Was there anything in British history, the history of the American colonies, US history of the north or south, that suggests anything other than imperial ambition and territorial expansion?
Since no one in the US could point to a history of peaceful co-existence, any assertion that the Confederacy intended to peacefully co-exist with the US, had little supporting evidence.
President Polk had temporarily settled the boundary with British North America in 1844. But that was acknowledgement of British military strength and economic power, rather than an indication of peaceful intent.