The most likely reason for any British alliance with the confeds would be so that they could stage their troops more conveniently for their next conquest.
The Confederacy actually sent an "ambassador" to the Court of St. James's to curry favor with Queen Victoria, but her ministers refused to even meet with him and snubbed him very publicly until he finally got the hint and went home.
Early on, the Brits had supplied guns, ammunition, and even warships to the Confederates, and when the Union forcibly removed two diplomats from an English ship, many in Britain declared it an act of war and demanded action against the North. But the crisis was defused when Lincoln freed the diplomats and apologized for the mistake.