That's not what gender reassignment means Pam.
Calling the protected characteristic 'gender reassignment' is a bit of a misnomer.
It means nothing other than your intention to transition. So telling your mum, your partner, or your best friend.
That's it. You're covered under equality law soon as you say it out loud.
Getting a GRC gives the protected characteristic of being a legal woman. (So sex).
It's quite important in terms of all women shortlists and prisons. But technically, not for much else. Although anecdotally, it is seen to be taken far more seriously than just the protected characteristic of gender reassignment.
It also means you can't challenge someone because you can't out them by asking for proof.
Neither of these two things require any kind of surgery or modification whatsoever.