The point was slavery was economically and politically viable in the American south far longer than Europe. It’s a reasonable argument to suggest Europe outsourced and profited from slavery, but it also wasn’t in peoples faces just as few people consider the working conditions of the 3rd world today when they buy stuff.
Anyway, the slave trade wasn’t that profitable for England. Profits where mostly split between Portugal, Britain, Spain, France, the Netherlands, and Denmark with no country having a massive advantage. Divide 12.8 million people across 400 years and then further split profits between multiple countries and it was a tiny rounding error for the European countries involved. It wasn’t even that profitable for the merchants due to the long trips and high risks involved.
PS: I don’t mean this to be apologist, more pointing out how minimal the gains required before people would inflict horrific amount of suffering.
You didn't actually mention that slavery was legal in many British crown colonies including the American colonies, which wasn't just "happy participation by merchants", but was day to day life for many UK subjects. "Slavery ended in the British Empire after the Slavery Abolition Act came into play in 1833", about 30 years before it ended in the US, a former British Colony independent less than 100 years at that time, a short time by any standards.
“Between 1808 and 1860, the West Africa Squadron captured 1,600 slave ships and freed 150,000 Africans. They resettled many in Jamaica and the Bahamas” and no that’s not a euphemism for enslaving them in Jamaica and the Bahamas.
You're of course only talking about local slavery in the UK. They were among the primary slave traders outside of their home territory. The British Empire and its prominent subjects traded in millions of slaves between 1400 and 1800. Centuries of profit from that slave trade helped fuel investment into industrialization across the UK.
This is a common escape for European apologists of slavery (whether in North or South America). Europe has largely gotten to avoid the consequences of their slave trade (they properly owe many trillions of dollars in reparations). The US history of slavery has nothing on the scale of European slavery - 12+ million slaves across the Americas (and that's merely the first generation number).