What's your POD? Given that Britain didn't want anything to do with the CSA IOTL -- even during the Trent crisis, most of the war party in Great Britain were against the idea of co-operating with the Confederacy -- going to war on their behalf seems unlikely, to say the least. You'd probably need a POD in which Britain felt threatened somehow by the USA, such that they judged splitting off a large chunk of American territory would be necessary to secure their interests. But the only way America realistically *could* threaten Britain would be by invading and occupying Canada. This, of course, is likely to make the Canadians quite hostile to the USA, so in any plausible "Britain allies with the CSA" scenario the Canadians are far more likely to be cheering Britain on than threatening to rebel.