A book worth looking at in answer to this question is Adam Hochchild's Bury the Chains which tracks the history of the political campaign to abolish the slave trade in England. It's a good read and also shows, I think, that there's not a simple answer to your question.

Britain in the eighteenth century was an unsentimental, largely rural society. I imagine the average Britisher at the time would have reacted to the proposal to abolish the slave trade as quixotic. There is no Biblical condemnation of slavery, for example.

I think much more historical research has gone into the abolitionists rather than the pro-slave traders. Added to this, average/unimportant people leave little or no historical record. So a definite answer would be hard to state.

The abolitionists were a minority and it is always a highly-motived and dedicated minority that get things done by relentless and unceasing campaigning. The late eighteenth century saw the rise of the Romantic and the Evangelical movements. The Romantics placed feeling above calculation and the Evangelicals burned with intense zeal for righteousness and had a highly self-conscious regard for morality. Thus the slave trade struck the abolitionist movement (largely Christian moralists) as an abomination. The abolition of slavery was the first great victory for this movement that would come, during the Victorian era, to be heavily influential. But to claim that they expressed average opinion during the eighteenth century would be a mistake.