Legally, in the UK, a person’s sex is entirely determined by their chromosomes, gonads and genitalia, where those three things are congruent, which they are in the vast majority of cases. In rare cases where they’re not, a determination will be made by the court after hearing expert evidence on those three things.
This was established in Corbett, and reaffirmed at various times since. I don’t understand why anyone thinks there is any doubt about that.
The outcome of the determination, whether straightforward (in the vast majority of cases) or occasionally difficult, is what is recorded on a birth certificate.
What some biologists, doctors, or anyone else has to say or not say simply isn’t relevant in law.
if at some time a case goes to the Supreme Court which substitutes an new way to determine a persons sex then that will override the Court of Appeal in Corbett. Fairly obviously a great opportunity to do so, if it were ever going to happen, would have been FWS, and that didn’t happen.
“Biological sex” wasn’t clarified in FWS because every judicial mind applied to the question already knows what it means. Which is what I said.