> Didn't the British empire ban slaves and slavery on English soil at the start of the 19th century?

Depends on your meaning and what dates. The British did NOT ban slavery "at the start of the 19th century" (meaning 1800s). They banned only the buying of slaves using English vessels and even then it was nominally enforced. In 1833, the English compensated approximately 3000 slavers for "loss of business assets" (yes, that's right, they compensated the slavers, not the slaves) including the ancestors of David Cameron, the Bazalgettes and other wealthy families.

Almost instantly after the passing of the 1833 act, the British used indentured labour to replace slavery. Indentured labour was found to be equivalently cruel. The historical record shows death rates as high or higher than the slavery period for indentured labourers brought from South Asia to the West Indies and South East Asia. https://www.theguardian.com/commentisfree/2006/dec/12/thefor...

The British banned slavery on the British Empire at the start of the 19th century. But the slave trade was banned on English soil way before that, in 1102. However, there were imported slaves in England, which got their freedom in the mid 18th century (via some court cases because slave trade etc was banned on english soil in the common law), effectively ending slavery as a whole in England. Then 50 or so years later, as the anti slave trade movement grew, this extended to the colonies and the whole empire.

It did:

https://en.wikipedia.org/wiki/Slave_Trade_Act_1807

https://en.wikipedia.org/wiki/Blockade_of_Africa

https://en.wikipedia.org/wiki/Slavery_Abolition_Act_1833

It even pressed other countries to abolish it (not only for pure reasons, also for commercial ones, but they'd still make Machiavelli proud).