With the forenote that I'm opposed to slavery, British companies and wealthy individuals purchased (or owned) copper from Welsh mines (or some other commodity) which they traded in Africa with African nations in exchange for humans (who were purchased already 'enslaved') and then either further traded in the Americas to plantations or used directly in plantations owned.
This was a large and complex business enterprise and didn't involve formal enslavement by the British Government.
The Abolition referenced above was the British Government outlawing slavery within its territories and by its citizens - as an act of Government it was only supported on the promise of slave owners (which included nice little old widowed grannies of modest means with a 2/6 th share in a slave left by an uncle) being recompensed for their forfeited property.
The funds that flowed to former British slave owners went on to establish canals, factories, rail lines, develop colonies in Australia, etc.
From that it follows that everyone whose property rights were created by the British government owed tax to Britain on that property. Ergo, the American Revolutionaries were in the wrong and the U.S. owes Britain substantial reparations for property theft and non-payment of taxes.
(Also, would be a bit odd for the British government to pay people who were not enslaved by it, but instead by private individuals)