I'm not sure I agree with 'In effect, [that] the BBC [decides it] knows better than the Supreme Court'.
The SC has ruled that sex means sex for equality purposes, but it can still be true that 'being transgender' (with or without a GRC or GD diagnosis) has legal or social salience (in fact it must have, given there is actual legislation on the topic).
Reporting a TW as a woman is inaccurate, but reporting him as a man is incomplete. Reporting him as a TW also needs further explanation for the portion of the population that think TWs are TIFs. There's a lot of information to pack in, but it's got to be done (and of course TRAs are furious, because these stories usually show TWs in a bad light, even compared to other men).
Use of names and pronouns is a separate issue. Any confusion as to actual sex can be mitigated with good information upfront, but I think the use of cross-sex pronouns by media is defensible in principle, provided it's consistent (no switching to male when they turn out to be rapist gender!). And it drives home to the reader how ludicrous it all is.
If 'being transgender' doesn't have the social convention of name and pronoun use, then it has nothing at all (well, apart from the dodgy ID, which also needs getting rid).
Not saying I agree, but it would be a self-consistent approach.