I don't think that overturning the Somersett ruling would be sufficient, though of course I don't deny that it would be useful. The thing about the British political system is that there is no equivalent to the US Constitution or the US Supreme Court: a body of law which can't be easily changed and which requires a careful, long and meticulous process to revise. British common law works on precedent, yes, but there is no precedent that cannot be overturned by a simple vote of Parliament (provided that royal assent is given, and in practice it always is).
If slavery is declared legal on English soil, there is still nothing to stop Parliament from abolishing it just as easily as it did IOTL. It's not as if there will grow up a major slave-holding culture to provide a strong lobby against abolitionism, as was the case in the American South; England isn't exactly prime land for plantations. At most, there would be a few rich people who had slaves instead of ordinary servants even though they could easily afford ordinary servants anyway—certainly nothing like the massive, cultural/'national' attachment to slavery which existed and thrived in the Southern United States.
When we consider the factors that caused the abolition of the slave trade and then of slavery IOTL—Anglicanism and the associated trend towards what one might call 'Christian moralism', the utter lack of any major pro-slavery interest in the decision-making part of the British Empire (whereas there was such an interest, a very strong one, in the decision-making part of the United States), the comparative lack of the places amenable to slavery as the Dutch and Spanish had which encouraged the preservation of slavery in those countries—these don't appear to be very easily variable.
I'm going to settle with 'difficult'.
I would disagree with others on the best path to accomplish this. I'd argue that what you need is some much greater British colonial success in the Caribbean sugar islands (look back to the Anglo-Dutch wars, perhaps?). Those were enormously profitable; half of one Caribbean island (Hispaniola) generated more income for France than the entirety of the Thirteen Colonies did for Great Britain. Temporary success as in the Seven Years' War wouldn't do it, because that was a massive disturbance to the equilibrium of the colonial balance of power which resulted, naturally, in Great Britain swiftly losing almost all its gains. We would need Great Britain to be accepted among the European powers as the owner of sizeable holdings in those regions. That might—might—make pro-slavery interests among the British upper class strong enough to seriously delay abolition.
The obvious problem, and one that I admit I'm not sure how to solve, is that such early PoDs would be fairly likely to butterfly the existence of the American Revolution, let alone its success, which depended on several battles that were far from predetermined. But perhaps there is a solution of which I am unaware.
Have a nice day.