I'd note, though, that beheadings have precedent.
There's still a large technicality that I think is being missed here.
If the law is illegal who do you sue? In the US you can challenge a law as unconstitutional when it's enforced by suing the enforcer. You can't sue the individual people who passed the law; just basically the cops when they apply it (or the government at large as a collective individual).
Eh, if you like.. the main problem with this kind of discussion is it's just so unthinkable that it's hard to really reason about.
But yes, strictly in principle, it is of course His Majesty's prison service, King's Counsel, etc.
I suppose what I meant really was that they are not today (practically speaking) in law-making or executive office.
Er, today. The UK monarch is completely immune from prosecution.
> parliament declaring a genocide of all Londoners,
This is only illegal insofar as it conflicts with ECHR. An explicit repeal of ECHR and withdrawal from the European convention on human rights would be required, and there are people agitating for that. Once that's out of the way it's entirely possible for Parliament to start legalizing open season on whoever they like.
The precedent is, of course, Northern Ireland, and the endless litigation since Bloody Sunday and similar incidents ("soldier F").