For whatever reason the Brits were the ones most open to Luther's teachings of the 5 solas. So much so that much of the rest of Europe often referred to them as "Protestant England". This encouragement that it's mainly up to the individual to further his own relationship with God led many to get closer to God, which led them to loving their fellow man more, which led them to wanting to get rid of slavery. It was an organic revolution pushed from the ground up, not a top-down imposition forced onto the culture by elites. The farthest you can find this is in the early 17th century, particularly among the Quakers on both sides of the ocean.
That England abolished slavery in 1833 when they were still very much an empire, even the top dog of the world, is quite remarkable. No other empire had done that. Not the Egyptian Empire. Not the Assyrian Empire. Not the Medes-Persian Empire. Not the Babylonian Empire. Not the Greek Empire. Not the Aztec Empire. Not the Roman Empire. Not the Ottoman Empire. The game changer was the English Empire. And it was a cultural thing, with other Anglo majority nations doing the same (U.S. a few decades later, and the newly starting Canada starting out slave free.)
And what did they do with their influence? After WW1 England's geographical footprint was the largest it had ever been (because the Allies let England control what had been left of the old Ottoman Empire after the OE lost in the war). The allies had asked various people groups within the OE to fight back against their oppressors in exchange for the Allies allowing the different groups to control their own areas -- hence the nations that exist today in the much of the Middle East. Before they changed hands, though, the Brits outlawed slavery in those lands for the first time in their history.
Meanwhile, England and the U.S. used their trade and military allegiances to discourage slavery in other nations, with the U.S. taking on more and more of the lion's share of the global abolitionist movement as the U.S.'s influence rose, after WW1 and most especially after WW2.
If you want to pick someone to give credit for the worldwide abolitionist movement, credit should go to God Almighty. But if you don't like doing that and want to pick a group of people to give credit to, then ironically it should go to the group that's most vilified for slavery as though we invented it instead of ended it.
Thank you.