Slavery is totally evil, but it should also be noted that it was relatively easy for Britain to have supported abolition of the slave trade and (later) slavery itself because it had no real need for slaves. Whether or not the British would have supported emancipation as early as they did if they had needed slave labor is an unanswerable question.

The industrial revolution and the enclosure process resulted in an excess of labor in the English south. There was no need to import workers, enslaved or otherwise. The opposite was true in the Sugar Islands (West Indies) where the local (Indian) population had long-since died off because of imported disease and transported English convicts and prisoners of war either died rapidly or escaped and melted into the "free" population, and in the Southeastern US. Agricultural products (rice, sugar and cotton) in these areas required a large and basically unskilled labor force, and laborers who were free to move on did so.

That is the basic reason for the use of slaves - there simply were no other workers available. The economy of the West Indies basically died in 1838 with the enactment of the British Abolition of Slavery Act, not to be revived until the tourist trade arrived over a hundred years later.

Whether slavery really ended in the American Southeast after the Civil War is very questionable. Former slaves were free to leave, but they had no real place to go and, if they stayed, their living and working conditions wouldn't change much for at least a generation.