The book I recommended in original comment (A World on Fire) will give you a good inside-look at the British political thought & decision making during that time, including Parliamentary debates & communiques between Palmerston and his ministers. Their motivations are complex, like any other politician, but if they were purely economic & geopolitical opportunists I would have expected them to leap at dividing America and keeping the cotton flowing. They didn't.
> I've generally had a pretty cynical view towards British abolitionism
I am not an expert on abolitionist movements or their political effects, but there was a strong moral outrage to slavery in the 19th century and it developed very rapidly, starting in Britain. I recommend reading about the British public's reaction to the publication of Uncle Tom's Cabin.
> their empire, whose 19th century wealth was bootstrapped on the profits of of slave-trading and the manufacture of raw materials extracted via slave labor, was uniquely-well-positioned to come out on top in a world order deprived of slave labor
You're not wrong, and you can toss colonialism in there, too. The Japanese were made the same argument in the early 20th century as they were trying to expand and acquire colonies and kept getting their knuckles whacked by the Europeans, who were (mostly) trying to pump the brakes on colonialism by that point.