Parliament doesn't have royal perogative, the Crown does which means the current Government, I believe it was last used by the conservatives during the Falkland crisis. My understanding is that the PM consults with the Monarch but doesn't have to act on the advice given.
Then in what way would that prevent a war? The Cabinet declares war and the Cabinet has royal perogative, so it's Cabinet alone which is required in order to declare war. (This didn't change until Tony Blair, who surrendered that to Parliament.)
As for my opinion on the British citizens not wanting to enter a war, I'm basing that on the British anti-slavery movement which was large and the protests carried out by those of the working class. I can't provide statistics without searching on line, I'm just doing the same as you, basing my opinion on what I've read.
There's a distinction between a (very real, but general) anti-slavery attitude and the specific case of the Trent.
Royal Navy Reservists volunteered in large numbers during the crisis:
'To the Shipping Master, Limehouse.
We, the Volunteers of the Royal Naval Reserve in the port of London, having heard that our flag has been grossly insulted by an American ship of war, and people who claimed its protection forcibly taken from it and made prisoners, we write this to let you know that we are ready to fulfil our engagement and protect the honour of our flag, our good Queen, and country whenever called upon to do so.
We respectfully request you will make this our determination known in the proper quarter.
Signed on behalf of the Volunteer Reserve Force at present in London. (Here follow the signatures.)'
'To Captain Luckraft, R.N.
Her Majesty's ship Hastings, Liverpool, Nov. 30,1861.
Hearing that our flag has been insulted, we, the Royal Naval Reserve men of Liverpool, would esteem it a favour if you will kindly represent to the proper quarter our entire willingness to serve and protect our gracious Queen and country and the integrity of the flag under which our fathers gloriously fought and conquered.
J.B. BECK, for the Liverpool Volunteers of the Royal Naval Reserve.'
'To Captain Palmer, R.N., Her Majesty's Ship Castor.
Her Majesty's Ship Castor, North Shields, Nov, 30,1861.
Sir,- Having heard that our country's flag has been grossly insulted by an American ship of war, and people who claimed, its protection forcibly taken from it and made prisoners, we write this to let you know that we are ready and willing, and that at the shortest possible notice, to protect the honour of our flag, our Queen, and country wherever and whenever called on, or any provocation given for us to do so; and we will do it with a right good will, a proof of which was given when the circumstance was spoken of on the Castor's maindeck this morning, by the whole of us joining with three hearty cheers, and three times three for our Queen and country.
Signed by the Volunteers for the Royal Naval Reserve on board. [Here follow the signatures.]'
'To Captain Heard,
Her Majesty's ship Trincomalee, Sunderland, Nov. 30, 1861.
Sir, - At a general meeting held on board Her Majesty's training-ship Trincomalee, now lying in the South Dock.
Sir, seeing by the Shipping and Mercantile Gazette the insult offered to the British flag by the Americans, we, the undersigned Naval Reserve men, do freely and spontaneously come forward and offer our service to resent the insult, and will shed the last drop of our blood against any nation who dares to insult our gracious Queen or national flag.
Signed on behalf of the meeting, over 100 present,
JOHN ROBERTSON, Chairman.'
'Whitehaven, Dec. 5, 1861.
Sir, - We, the seamen of the Royal Naval Reserve in this port, have heard with indignation that our flag has been insulted by an American ship of war, and we write this to let you know that we are ready to fulfil our engagement and protect the honour of our flag, our good Queen, and country, whenever called upon to do so.
(Signed on behalf of the men of the Royal Naval Reserve, 38 in number.)
To Captain Comber, Inspecting Commander, Coastguard.'
'Maryport, Dec. 7,1861.
Sir, - We, the undersigned members of the Royal Naval Reserve at Maryport, do hereby signify to you our services in any action taken by the Government to avenge the insults offered to our flag, in the event of the American Government refusing to make the necessary reparation and apology for the stopping of the Trent and arrest of some passengers on board.
(Signed by all the Reserve, 20 in number.)
To Captain Comber, Inspecting Commander, Coastguard,'
'Peterhead, Dec. 10,1861.
Sir, - We, the undersigned men, enrolled in the Royal Naval Reserve in the port of Peterhead, considering that a most wanton and unjustifiable insult has been perpetrated by the commander of the war-ship San Jacinto, belonging to the Federal Government of America, on the British flag, hereby most cordially make offer of our services, should Her Most Gracious Majesty see fit, by Royal proclamation, to call for them.
(Signed by 73 Royal Naval Reserve Men.)
To Commander Mould, R.N., Inspecting Commander, Coastguard, Aberdeen.'
'The Tyne Naval Reserve. - Shields, Wednesday [4 December 1861]. - This forenoon the letter from the Lords of the Admiralty to Captain Palmer, of Her Majesty's ship Castor, in acknowledgment of the address sent from the Tyne by the Royal Naval Reserve Force, tendering their services to the Queen in case of war with the Northern States of America, was read from the quarterdeck of that vessel to the Naval Reserve men, and was received with immense enthusiasm. The letter read was the same as that published in The Times on Tuesday. After the reply was read to the men they determined to have a demonstration in the seaport of Shields that afternoon, and at half-past 1 o'clock they mustered in strong force upon the New Quay, North Shields, as fine a body of young fellows as it was possible to clap eyes upon. The officers of the 1st Northumberland Artillery kindly put their fine band at the service of the men, who had mustered an immense number of union jacks, ensigns of St. George, &c, and when the procession was formed it had quite a warlike appearance. About 2 o'clock the band struck up "Hearts of Oak," and the men proceeded to march through the principal streets of North and South Shields. They were met everywhere with the greatest enthusiasm by the seafaring population, especially in the neighbourhood of the quays and shipping. Above 1,000 seamen are now enrolled; in the books of the North Shields Shipping-office as Naval Reserve men.'
From the Times:
Soon after noon to-day a private telegram was received in Liverpool announcing the boarding of the Trent by a Federal vessel of war and the forcible removal of the Southern Commissioners. The intelligence spread with wonderful rapidity, and occasioned great excitement among all classes. On 'Change the utmost indignation was expressed, and in a very brief space of time the following placard was posted:-
"Outrage on the British Flag. - The Southern Commissioners Forcibly Removed From A British Mail Steamer.
"A public meeting will be held in the Cotton Sales-room at 3 o'clock."
In compliance with the preceding announcement a meeting was held in the Cotton Sales-room at 3 o'clock, which was crowded to excess by nearly all the gentlemen frequenting the Exchange. The meeting was quite as remarkable for enthusiasm as numbers. After several gentlemen had been requested to preside the chair was occupied by Mr. James Spence, and on taking the chair he proceeded to read the subjoined resolution:-
"That this meeting, having heard with indignation that an American Federal ship of war has forcibly taken from a British mail steamer certain passengers who were proceeding peaceably under the shelter of our flag from one neutral port to another, do earnestly call upon the Government to assert the dignity of the British flag by requiring prompt reparation for this outrage."
On hearing this resolution read the meeting expressed in a most unmistakeable manner the feeling by which it was pervaded in favour of the views included in it. When silence had been in some measure restored, the Chairman remarked that when the news of the outrage reached this town the feeling created was one of surprise, mingled with indignation. He remarked that we had all heard of the sacred dignity of the American flag. That dignity, he proceeded to say, was a means by which the persons engaged in the nefarious slave trade could at once protect themselves by hoisting the American flag, which fully enabled them to resist any attempt to search such vessel. He trusted it would not be allowed that men prosecuting so nefarious a trade should be protected, and that men peacefully proceeding on their own affairs, under the protection of our flag, might be forcibly taken out of our ships. (Cheers.) On the contrary, he believed that the people of this country would not by any means permit such an outrage. (Cheers.) He said, in having agreed to take the chair on this occasion, he did so without reluctance or regret, and he felt deeply that he only expressed the feeling, not merely of the meeting, but of the community in general, when he said it was the duty of the people to press on the Government the imperative necessity of vindicating the honour and dignity of the British name and flag. (Loud and continued cheering.)
Mr. H. C. Chapman, as a mere matter of form, moved that the resolution be adopted.
Mr. A. Forwood said he felt much pleasure in seconding the adoption of a resolution which must find an echo in every English bosom.
Mr. John Campbell while fully concurring in the propriety of preventing any outrage from being offered to the British flag - a sentiment which was universally acknowledged throughout the kingdom - said he felt assured that there was no Englishman, Irishman, or Scotchman who would not at once, and promptly, resent any insult offered to our flag. (Cheers.) "While feeling this in the strongest manner and to the fullest extent, he considered that there still remained some reason to doubt whether the facts related, and acted on by calling this meeting, were in reality a breach of international law. (Cries of " No, no !") He referred at some length to the opinions of the law officers of the Crown, as being in some measure inclined to show that such a step as that taken with respect to the Southern Commissioners was justifiable under the existing state of international law. In conclusion, he proposed a direct negative to the resolution. As, however, he was not desirous of doing anything which would create a spirit of dissension, he was willing to adopt any middle course which could be suggested, and urged the propriety of postponing the consideration of the subject till to-morrow (this day).
The Chairman suggested that, to meet the objection thrown out by Mr. Campbell, it would be sufficient to strike out of the resolution the words "by requiring prompt reparation for this outrage."
Mr. Campbell said he could not concur in the suggestion of the chairman, and must decline to do so.
Mr. Torr expressed his concurrence in the views put forward by Mr. Campbell, and in doing so met with frequent interruption. He argued that the present meet was hastily convened, and had in its proceedings already prejudged the case, with the merits of which the meeting was unacquainted. He insisted that there was no reason to believe that the responsible Ministers of the Crown would allow any insult to be offered to the British flag. (Loud cheers.) He urged the advantage of proceeding calmly in considering a case such as the present, which, if prematurely urged to extremity, might result in involving this country in a war. (Great interruption.) He contended that to urge on the Government a particular line of conduct in respect of the proceedings now under consideration was impolitic and unjust. He would not, and no Englishman would, advocate putting up with insult; but in the present case, let him ask, what had the Americans done? [Mr. Chapman. - They fired a shot across the bows of the Mail Steamer to bring her to, and as she did not stop for that they fired a shell at her, which burst close by her. (Tremendous cheers.)]
Mr. Torr proceeded to say that there was every reason to avoid coming to a hasty resolution, and, in thanking the meeting for the patience with which they had heard him (loud and ironical cheers), he again urged on those present to consider the matter calmly and dispassionately, and not to be carried away by the impulse of feeling in a case which required mature judgment and calm deliberation. A letter had been shown to him by a Southern gentleman, in which it was stated as a positive fact that the law officers of the Crown had, in anticipation, expressed a decided opinion in favour of the legality of a proceeding similar to that which had just taken place in regard to the Trent by the San Jacinto.
Mr. J. Turner next attempted to address the meeting to the same effect as had been done by Mr. Torr and Mr. Campbell, but the feeling of those present was so decidedly opposed to that view that he was forced to desist.
The resolution, as proposed to be amended by the chairman, was then put to the meeting, and carried by a tremendous majority, and amid the most deafening and enthusiastic cheers. For the negative only a few hands were held up.
At the conclusion of the meeting, which was at 4 o'clock, a number of the older merchants on 'Change expressed privately their conviction that the meeting and its proceedings had been premature.
Note that those who are supporting the "this isn't worth a war" side of the argument are being shouted down by a huge majority of those present.
The same is true of politicians:
Benjamin Disraeli:
'The passions of the people are very high at the present moment, and if the Ministry chose to send fifty thousand men to Canada they would be supported.' (
source)
'I am a man of peace, but if we are attacked, the honour of England must be asserted and vindicated.' (Birmingham Daily Post, 12 December 1861)
He wasn't alone. Of the MPs who made speeches during the crisis (
from):
Support war in the event of reparation not being made
W. R. Seymor Vesey-Fitzgerald (Conservative, Horsham)
Col. Walter Bartelott (Conservative, West Sussex)
Mr Stephen Cave (Conservative, New Shoreham)
Sir Robert Clifton (Liberal, Nottingham)
Charles Newdegate (Conservative, North Warwickshire)
Lord Fermoy (Liberal, Marylebone)
John Harvey Lewis (Liberal, Marylebone)
John Laird (Conservative, Birkinhead)
William Cox, (Liberal, Finsbury)
Sir William Miller (Liberal, Leith Burghs)
Edward Horsman (Liberal, Stroud)
William Williams (Liberal, Lambeth)
Henry Bouverie William Brand (Liberal, Lewes)
John George Blencowe (Liberal, Lewes)
Benjamin Disraeli (Conservative, Buckinghamshire)
John Morgan Cobbett (Liberal, Oldham)
Frederick Peel (Liberal, Bury)
Henry Brinsley Sheridan, (Liberal, Dudley)
Harry Foley Vernon, (Liberal, East Worcestershire)
William Coningham (Liberal, Brighton)
James White (Liberal, Brighton)
Oppose war in the event of reparation not being made
John Bright, (Liberal, Rochdale)
William Edward Forster (Liberal, Bradford)
Alexander William Kinglake (Liberal, Bridgewater)'
There were, in fact, no mass anti-war demonstrations in the United Kingdom during the period of the Trent. There were no mass pro-war demonstrations either, of course, but there were such things as large numers of men enrolling in the naval reserve to the cheers of the crowd.
This was also evident to people not from the country. One American visitor to the UK wrote to William H. Seward declaring that "The people are frantic with rage, and were the country polled I fear 999 men out of 1,000 would declare for immediate war."
The case of Trent is so transformatively different from the general British opinion of the ACW that it's almost irrelevant to bring up general ACW material during the Trent period. You can find one or two MPs and the occasional person who opposes war if the Trent ultimatum is rejected, but the overwhelming majority of evidence - from the ultimatum to the war warnings to the public meetings, the mass sign-ups, the views of Cabinet and Parliament and of private US citizens visiting the UK - suggests that there would be no major impediment to a war during Trent.
I should also note that, with Bright being one of the few anti-war MPs, even his own constituency didn't agree (also from the Times):
What we think of arbitration in such a case as this the Americans may gather from the proceedings yesterday at a public meeting in Mr. Bright's own borough of Birmingham, where the friends of arbitration were beaten at their own meeting by an overwhelming majority. We wish we could show them in an equally public manner what the holders of American securities think. That they hope for peace, even as the Birmingham men hope for peace, and as we all hope for peace, is emphatically true; but no one of them would put his American securities, whether valuable or worthless, in balance against his feelings as an Englishman, nor would desire peace any otherwise than so far as it is consistent with the honour of his country.