Actually forget I ever said that. I’m not really sure what I was thinking of.
Maybe Granville Sharp (1735 - 1813) who is he was not the father of the UK abolitionist movement, certainly dipped his wick. And Sharp's dad was Thomas Sharp (1693–1759), Archdeacon of Northumberland, so there was probably a bit of the old time religion around.
And twelve hundred years before Pope Gregory had walked through a slave market and exclaimed "Non Angli sed angeli", which is Latin for not slaves but Anglicans.
So it looks as though stammerhead is right.
Christians didn't outlaw slavery when they took control of Rome.
William I abolished the slave trade with the result that slavery as such disappeared in England in the 12th century. And since the Conqueror invaded England with the blessing of Pope Alexander II, the church could claim some credit for the eventual abolition of slavery.