The 1992 regulations don't create rights for employees as individuals, but impose obligations on employers - to provide certain facilities. Nothing is said about policing use of the facilities.
Of course it was 'obvious' what was meant. But I think trans people could claim established rights, because they would always use the facilities of their acquired gender, if they could get away with it. And because they were rare, timid, and made a bit of passing effort, they did get away with it.
The EA2010 does provide exemptions and exceptions that could enable true sex-segregated provision (Sex Matters has lots of information), but providers just don't do it, because #bekind/expense/whatever.
There's a good explanation by NC on Sex Matters of the legal import of a GRC. Now that tax, pensions, and marriage have been sorted, virtually the only significance is for identifying a comparator in discrimination cases. So, eg Upton would say excluding him from the women's changing room would be discriminatory vis-a-vis both sex and gender reassignment, and this applies whether he has a GRC or not. It's not about the exclusion per se but the resulting lack of 'suitable' provision, meaning he is disadvantaged compared both to a 'cis' person of the same legal sex, and to any person of the opposite legal sex.
It doesn't make him female for evidentiary purposes. If an Orthodox Jewish or Muslim nurse complained because they've got nowhere to change (man in the changing room), the man having a GRC wouldn't fix the problem. I would love to see that argument play out.