While the Europeans of the 17th and 18th centuries had a deep cultural discomfort with outright chattel slavery, and were very ready to do away with it, they mostly did not. The Somerset decision was in 1772, Vermont banned slavery in 1777, Pennsylvania's gradual abolition law passed in '80, Massachusetts courts ended local slavery in '83, Denmark forbade the trade in '92, and the French Republic abolished the institution in '94. Philosophically, they were there. But fate, chance, and short-term enthusiastic radicalism shaped early decision-making on the question, not reasoned debate and legislation. It's no accident that 2 of those were imposed by courts, 1 by royal decree, and 4 came about in the midst of revolution. Only the most radical legislatures were willing to act.
Where slavery could be put "out of sight, out of mind," it was. Where there were legal or economic obstacles to it, they tended to win out... in the short term. So it was a bit of a surprise to do a quick search of the forum and find out this never seems to have been seriously considered before. I recall reading a completed short story without Somerset years ago, but off the top of my head I can't think of a single timeline that goes through the period that significantly explores even a different Somerset decision.
In what circumstances could the Somerset decision be outright averted?
There may be more answers, for example simply having a less sympathetic judge than OTL Lord Mansfield, but there's one straightforward path to get there: have a law on the books delineating the rules for slaves in England.
Lord Mansfield was apparently going out of his way to not create precedent, but "there's no law that allows slave-catching" was sufficient in the absence of a law. Given how cautious courts were in tiptoeing around controversial decisions, nothing remotely like Somerset will arrive by way of the courts in the face of established law. If legislation sets rules of any type for resolving the situation of escaped slaves, the OTL ruling becomes impossible.
Even if a law set rules
hostile to slavery, those rules being committed to paper would intrinsically confirm slavery had a place in England, and that the question was
how much of a place. If you guarantee the rights of slaves to testify in pursuit of their freedom, for example, you also guarantee the right for slave owners to legally own people - slaves can't testify if they aren't slaves. If you state that people are released from slavery if they spend a year and a day on English soil, then the court system is committed in part to supporting property rights in people for a year. Almost any conceivable law on the question would strengthen slavery relative to OTL's Somerset principle.
Working backwards, and trying to remain straightforward:
3. A law of this type might be the path of least resistance to an England hosting some slavery. Again, the freedom principle could be absent from English common law for other reasons, but this is the simple scenario.
2. Likewise, many paths could lead to such a law, but I suspect the single most likely context would be American representation in Parliament. If parliamentary leaders were making concessions to American supporters or factions, it's very probable that laws accounting for the movement of slaves would have been part of that.
1. Dozens of threads have discussed how or whether Americans could get seats in Parliament. Rather than get lost in the weeds inventing a unique scenario like
@Thande or debating the plausibility of a 1760s POD, I'll just resort to OTL and Lord Baltimore.
Several English colonies were established with "lord proprietors" from their outset, men who were given titles to go along with their charters to territory in North America. Mostly, though, this was nothing like giving representation to the colonies. These were English titles explicitly relating to locations in England, they weren't a ticket into the House of Lords (or Commons), and in many cases they were lower titles than ones the bearer had already held. Since colonial charters had little long-term power to reserve rights to lord proprietors, the titles were not very meaningful.
But there was the Lord Baltimore exception. His titles, first to Avalon and then to Maryland, were Irish peerages that had established rights in Irish and English law. Not only could the king create them at will, but the bearer was entitled to sit and vote in the House of Commons in England. New World seats in Parliament actually existed. Only one person received them, and neither he nor his heirs ever availed themselves of the privilege in London, but it happened in OTL.
Memory serving, officially both titles were created under the rules for counts palatine, which were by precedent entitled to assume responsibilities locally that were usually reserved to parliament or the king. Some or all later lords proprietor were legally considered to have their powers in the same sense, but none of them held Irish peerages. In the long run it made for relatively meaningless titles.
So
1, the POD. As IOTL, American Irish peerages are granted to friends of the king who seek involvement with colonial ventures during the 1630s. Unlike OTL, this is normalized as the default for lords proprietorship in the Americas. With the Restoration in the 1660s, peerages paired with (mostly theoretical) colonial privileges are handed out liberally to friends of the Crown. Some American-'Irish' peers have participated in government, some have settled in the colonies, some have done both, and a few have even gone to Dublin. They don't
represent America, but do begin to advocate interests
related to America (and the Caribbean), notably when Carolina proprietors cry out for support against Spanish-allied Indian tribes. Dissenter-founded colonies in OTL New Jersey, Pennsylvania, and Delaware are the first to transition to backhanded representation, sending title-holders to London where relatives, wealthy neighbors, and colonial governments support them in exchange for their votes and correspondence on colonial issues. As these practices spread and become more formalized, having a peer (or having as many as possible) is taken to be indispensable.
In Parliament these peers communicate American culture and interests to a certain degree, though mostly those of the American elite. They participate in debates and parties, or don't. Mostly they are fairly normal members.... but a lot more of them own slaves than normal. So
2, some time between 1690 and 1750, the legal status of slaves is addressed. By mid-century, as in our TL, there are thousands of slaves in England. By the end of century the number will be a little higher, and
3, no Somerset-type decision will be possible.
That doesn't mean the English will
like slavery, or that the future of slavery in Britain will be assured. Far from it! I'd expect anti-slavery agitation to be a bit more popular, earlier. But where we had a bedrock certainty in the absolute freedom of whole regions of the Earth,
this England would be in the same boat as pre-revolutionary New England, New York, and Pennsylvania. Abolition is almost certain, but they'll have to work for it.