The British didn't invent the African slave trade, and neither did Europeans, but the British became enthusiastic slave traders and exploiters of slave labor. The conditions in which such slaves were transported and worked were incredibly awful. Few slaves survived more than a few years on the Carribean sugar plantations, for example.
But the British were also world leaders in abolishing slavery. The humanitarian impulse that led first to British abolition had precursors but took root in the evangelical Christian movement of late 18th century Britain. Once the Atlantic slave trade had been abolished, Britain's next great expansionary impulse was driven increasingly by the evangelical impulse. These explorers were if anything, even more intrepid than their pirate predecessors.
Their altruistic intentions were not entirely matched by their effects. Plunking down freed slaves in new African communities far from tribe, clan and the ecology of their birthplaces was usually disastrous. Their forthright ambition was not only to save the souls of Africans but to replace their culture with English culture, superiority of religion and culture both being obvious to them. This too was frequently calamitous, at least in the short term.
Many of them paid with their lives, and frequently with the lives of their families.
In New Zealand, a missionary who attempted to persuade a Maori chief from war was punished by being hanged and shot by the chieftain, who followed up by drinking his blood and eating his eyeballs.
The slave trade to India, Arabia, and Persia continued long after the Atlantic trade had been suppressed, and British explorers encountering it were shocked at its horrors. How could such a change of mind have occurred in the space of the couple of generations since their ancestors had conducted a far larger trade in West Africa? Partly, I think, it was a real change in societal word view, and partly it was the fact that slave trading attracted a different kind of man than missionary work. But consider the case of John Newton, successively sailor, slave, slave trader, Anglican priest, and abolitionist crusader (and author of, among other hymns, Amazing Grace). Human's minds change one at a time.