That Mansfield ruling

The Mansfield ruling didn't actually make slaves free in England, though the perception among pro-slavery and anti-slavery lobbies was that it did. So, technically speaking, slavery wasn't actually abolished in England until very recently, I think.

It's a mistaken assumption made by quite a few lesser historians, such as William Hague in his much-heralded and over-rated biography of 'Wilberforce'. But I refer you to Simon Schama's excellent 'Rough Crossings':

"Although it is quite true that, in the interests of a clear-cut moral and legal drama, the press and public opinion in London had all taken the freeing of Somerset to vindicate Serjeant Davy's axiom that 'as soon as any slave sets foot on English grounds he becomes free', that was not, in fact, what Mansfield had said; indeed, he had inflicted contortions on himself to avoid saying it. What he had said was that the power of a master to TRANSPORT his slave against his will, out of England and to a place where he might be sold, had never been known or recognised under Common Law. And that, indeed, was the ground on which Somerset had been liberated.

"But aside from the exceptionally attentive, neither party - neither the West Indian sugar interest, which had now launched a furious lobby for legislation to recognise their property rights when in England, nor the elated crusaders for (African) freedom - took the measure of Mansfield's fastidiousness. Both sides did, in fact, think that he HAD made slavery illegal in England. Many owners continued, nonetheless, to act as if the SOmerset judgment had never happened. Auctions and sales were advertised and held, not just in London but in the provincial centres of colonial trade. Runaways were still hunted down." p61