Gender reassignment protected status can't be made contingent on medical treatment, any more than the issuance of a GRC can, because that would be an Article 3 violation.
I'm arguing to keep the protected characteristic, mostly for pragmatic reasons. But we need to keep quashing the lie, that it is anti-trans discrimination to treat someone in accordance with their sex, in situations where sex matters scientifically, legally, medically, or for safeguarding, or to refuse to give someone an unethical medical treatment just because they demand it.
If we take the two extreme cases, of a medically transitioned completely passing male transsexual who is not out, and a privately gender-questioning man who has taken no further action yet, the following might be discriminatory even though not targeted at them:
An openly hostile and mocking workplace culture towards transgender people.
Single-sex multi-user facilities only, incorporating disabled provision within them.
A sexed dress code based on birth sex.
(Of course, 'not out' guy probably doesn't pass, and might be treated less favourably on that account, which would be tricky to detect: my own experience of going shopping with a transwoman friend is that he receives the most obsequious customer service I have ever seen in my life.)