I agree with Plane.
Even if the decision was logical, it was not only unjust, it was a major cause of the Civil War, which could and should have been avoided. I don't think that slavery is or ever has been logical, but there was a lot of precedence for it, and English law has always been based on precedence.
The English ended slavery in 1833, and broke that precedence. Of course, slavery was less important to the English as a moneymaker, and American slaves in factories that did not require heating would have been able to undercut British cotton mills that were in a rather chilly part of Britain.
Justice was on some British minds, but they also had their financial motives.
Brazil was always the major slave labor economy, and the British felt that their dominion over the seas could be threatened by the Portuguese and Brazilians, especially when the Spanish and Portuguese crown were united from 1580 to 1640.