Bear in mind that the expansive definition of "woman" as bio + legal women is going to be challenged in the Supreme Court. But even with the Haldane/Inner House judgement, you can still discriminate on the basis of sex if it's a proportionate means to a legitimate aim.
I would say the Tickle case has demonstrated aptly how a single-sex service in the UK can never be achieved where the definition of "sex" in single-"sex" service means legal/certificate sex.
The aim of such a single-sex service for women can only ever be to provide for women (biological, but not those who hold a GRC) PLUS men (biological who hold a GRC).
Men with GRCs cannot be excluded on the basis of sex because their sex in law is woman (there are no grounds to exclude a woman from a women-only service), and cannot be excluded on the basis of gender reassignment because this has just been shown to be indirect discrimination.
And if anyone thinks there is some way to exclude a man with a GRC on the basis of gender reassignment to achieve a single-sex service for women (biological, which includes women who hold a GRC), then you've just admitted the "sex" in single-"sex" services, is defined in the Act in terms of biology, and not legal sex.