You don’t choose a gender identity. You know if you’re a man or a woman.
The list of gender identities and attendant pronouns continues to grow. We have people like Pips Bunce who change their gender identity daily. We have OMAB(1) people who self-declare as non-binary and then later self-declare as women. We have young women who transition to become transmen and then detransition back to self-acknowledged womanhood. We have gender critical people saying, in good faith, that they don't have nor understand a gender identity. We have genderists who describe themselves as "agender".
I would argue that people often do not know their gender identity. If they did, "gender fluidity" wouldn't be a thing and people wouldn't desist or change between identities.
Wasn’t it the ONS (or EHRC?) that said the relatively small number of transitioned people won’t affect the validity of this sort of health planning from a census sex question.
That's assuming that the numbers of GRC holders won't shoot up if self-id becomes law. (Pro-tip: it will.)
(1) Observed male at birth.