If you are considering how to answer, use the sex recorded on one of your legal documents such as a birth certificate, Gender Recognition Certificate, or passport.
Surely sex is the same on GRC and birth certificate - isn't that the whole point of a GRC
. Why they can't just put "sex on birth certificate" I don't know.
Also, I just checked, and you can't just self-ID for a passport - you need a GRC or a letter from doctor. So that means you need to have at least made the effort of convincing a doctor.
At least the sex question isn't entirely self-Id.
Will it be possible to correct disaggregate the data by cross referencing with GRC / passport sex change requests?