The link I posted says this
The guidance sets out how it would be lawful for a transgender person to be asked about their biological sex, in order to preserve single-sex spaces. The decision to ask could be based on, among other things, their appearance.
If there is doubt about whether they are telling the truth, other matters would then need to be taken into account when deciding whether to exclude them.
I think that the guidance covers situations where providers don’t believe the identification, knowing that identification can be changed.
It also says this:
It means that if a trans person is admitted to a service which is for the opposite sex, it would no longer be a separate or single-sex service, which could amount to unlawful sex discrimination.
If this is stated a clearly as this in the guidance, that would be a good thing.