So there was legal segregation in Britain until when? Isn't that the relevant question? 12.5 million black Africans, mostly men, were sold by Africans to Europeans in the transatlantic slave trade and 11.5  million blacks, mostly women, were sold in the trans-Saharan slave trade to North Africa and the Middle East. And millions more enslaved in the Mediterranean or in the Ukraine and the Caucasus and sold in the Mediterranean. (Cambridge World History of Slavery, Volume 3). And untold million slaves exported from India to central Asia and the Middle East---a much under explored topic. What stands out in human history isn't the existence or persistence of slavery, despite  universalist, fairly egalitarian religions like Christianity, Islam, or Buddhism.  What stands out is that slavery and the slave trade were ever abolished and suppressed and for that Britain deserves special credit. There is no reason for believing the rest of the world would ever have done so on its own.