> 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.
I've generally had a pretty cynical view towards British abolitionism, which is that the elites only disliked slavery to the extent that they found capitalistic imperialism (i.e. systems that controlled the lower classes through persistent indebtedness and restrictions on ownership of real estate) to be more profitable and politically stable than the systems constructed on the institution of slavery.
And, coincidentally, that their empire, whose 19th century wealth was bootstrapped on the profits 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.
In other words, they opposed slavery on the basis of limiting economic volatility, and maintaining their geopolitical status, more so than any moral basis.
I'd love to read more sources that balance out my cynicism. :)