OK, I understand the distinction you're making. Probably not the best choice of words on my part. (I initially thought to say that the court held Johnson's recommendation for an extended prorogation to be ultra vires, which is how I interpreted its use of the word "unlawful," but settled on what seemed at the time to be clearer language.)
Chris, I think ultra vires are, to mix languages, the mots justes. His advice has been deemed unlawful because its effect has been judged to be unjustifiable.
But unless I'm missing something else, it still seems to me that the Supreme Court has taken a step with this ruling in the direction of American-style judicial review. The court's decision is self-enforcing, is it not? No further action by the Prime Minister or Parliament is needed to give it effect; indeed, the Speaker immediately announced that Parliament would sit on Wednesday. So by its "declaration," the court has effectively nullified (or reversed or countermanded—I don't know what term would be most appropriate) a real-world action taken by the executive. At least, that's how it appears on this side of the Atlantic.
I don't know enough about American JR to comment on that. For myself, I think it is a dangerous decision because it involves the court's weighing the effect of a decision against the justification for that decision; and that seems to me to be supremely a matter for politicians and not for the judiciary. But, to misquote, I am not now nor have I ever been a Justice of the Supreme Court (and nor will I ever be, but that's another matter). What it does not do, despite attempts to present it as such, is to decide that Johnson "lied" to the Queen, or even "misled" her.
An oral constitution is not worth the paper it's printed on.
Possibly true, but utterly irrelevant. I wrote that we have no codified constitution, not that it was unwritten. Which part of "common law, statutes, conventions and practice", do you imagine is not written down?
Good op-ed piece by Lord Sumption, an ex-SC justice
Sumption's piece is, as one would expect from a phenomenally intelligent man, an eminent historian as well as a jurist, lucidly written; and its tone is one of sorrow, not of anger. The penultimate paragraph you quote is a superb summation. I suspect he shares my fear that this is step too far, even if he is prepared to support it as being, in the circumstances, necessary.
Jeremy