Slave-trade is not the same thing, and more shameful, than slavery.
Slavery is an extreme limitation on freedom, but a humane slave-owner may treat them correctly and particularly, not sell separately members of a family as often done in... well, I won't polemic on nationalities so, I would say, in some of the West Indies rather than in others.
While trading humans is shocking per se, to any Christian. And though Europeans rarely made slaves themselves, they knew perfectly that the African tribes who sold them slave, had captured them only to sell them.
So, many European countries, in the Regency era, had banned making new slaves, without freing existing ones. Yet slave-trade did still exist, and Sir Thomas may have had very sad anecdotes to tell, or more hopeful stories, without supporting slave-trade, because owning slaves didn't mean supporting slave-trade.
Such position could be met with rather often but I particularly remember General Lee, though it was half a century later. He owned a plantation with tenths of slaves and was better with them than most of his neighbours (and Virgina was much less hard for slaves than other Southern states), yet he wondered how to spare slavery at once and hoped it would be banned some day, but not immediately. If I remember well...