I think the British shouldn't be able to continue the war in Canada... Invading North Africa after France surrenders is already a pain in the ass, but getting to Canada is even worse. Maybe you could instead make peace with the British after invading the isles and take away most of their colonies, but the king would still rule in Canada as an exiled government and would have claims on Britain, and that would stop the war. Having France continue the war in Africa isn't so bad, but at least stop the British from being able to continue the war in Canada.