Once Lincoln issued the Emancipation Proclamation, the war became about
slavery. Did that matter to the British?

That matteresd to the British as the British didn't want to get involved with American politics and the Civil War as the union had a good reason to fight.

it is the dominance of   the spanish language and roman catholic region in mexico, french being the official language of several african countries, and the existence of a western-style government in south asian countries