OTL, the pro-slavery lobby was able to delay the end of the slave trade and the abolition of slavery in the British Empire for years due to their wealth--at one point, they paid for many MPs to go to the opera rather than vote on abolition measures.
In OTL, their power came from the West Indies and sugar production there.
In a TL where the American Revolution never happened or was crushed, might it be plausible that the planters of the OTL American South could join their wealth and power to the pro-slavery West Indies lobby to keep the slave trade and slavery going within the British Empire for decades longer than OTL.
How plausible is that? It makes sense, given the sheer wealth the hypothetical uber-slave-lobby would command. Plus churches in the OTL American South developed religious justifications for slavery that could be exported to Britain to counter Wilberforce and company's Christian anti-slavery arguments.
Bonus points if the Revolution is crushed and "Enlightenment Humanism" is temporarily discredited along with it so TTL's British Empire, nobody sees any moral conflict between desiring freedom for themselves and denying it to others and anti-slavery agitation is considered seditious libel (in Britain but not in the U.S., true statements could be libel if they allegedly threatened the public peace).