Why would slavery be completely banned in Britain first?

They had used slavery for many years but compared to other countries at the time they didn’t really rely on slavery so when some enlightened Englishman showed the cruelty of it the English people were not hard to convince that slavery should be removed by the British first/also they saw themselves as a big empire at the time and to ban slavery they would seem much better compared to other European empires at the time

Of the revolutions of 1848.

B. marbury v madison