I think basically the Somerset case merely pointed out there had never been any English statute law defining who was slave and who was free - legally it was a grey area, and those who believed English law supported the rights of slave-owners as established elsewhere might be in trouble. Even after Somerset some slave-owners, in the absence of a clear legal framework, tried to carry on as if slave laws made abroad applied in England. But public opinion was moving strongly against the idea slavery in England could be legal. See the 1825 law case involving a Mrs Storer, who was prosecuted by her Jamaican servant Philip Thompson after he alleged she ordered another servant to flog him. (https://www.ucl.ac.uk/lbs/person/view/43710) Although that reference oddly suggests Thompson completely lost his case, in fact he brought two prosecutions, one against Storer and one against her steward who carried out the assault. The steward was convicted but the case against Storer failed. The news reports of the trial focus on the facts - was he flogged, and at whose instigation? - and not at all on 'did she have a legal right to order this'. It's assumed from the outset that Thompson is not a slave, no matter his original status in Jamaica.

I could have saved myself a lot of typing by directing you here: http://www.mirandakaufmann.com/common-law.html