@HipTightOnions
Restricting by gender =/= lawful. It has no basis in law.
Is it actually unlawful though? Can providers who choose not to categorise by sex instead choose their own categories, as long as they are not discriminating against a PC?
That's a fair question and the answer is, yes, if they are not discriminating against a PC, they can categorise in any which way they like.
However, if a service excludes people on the basis of not falling into their chosen category, it may in practice be impossible not to discriminate against people with a PC.
Take gender. It's not a PC. There is no protected group sharing this characteristic. But an organisation says uses it anyway as a category to offer its services only to people who are defined as having "woman gender" by the organisation.
They therefore allow access to female clients who don't identify as men and to male clients who don't identify as men.
They exclude female clients who don't identify as women and male clients who don't identify as women.
Along comes a man who wants to access this service. Say it's a fully funded training program in STEM aiming to increase the percentage of women working in STEM and he has had challenges in his own life that means he certainly would benefit and he couldn't afford the training by himself.
The organisation refuses him access saying their places are limited to women and he doesn't meet their criteria for inclusion. He sues for unlawful discrimination on the basis of sex because the organisation allows other males access who are legally and biologically male just like him and therefore not women according to the relevant laws.
(The organisation cannot justify his exclusion by saying it's on the basis of him not sharing the PC of gender reassignment, because they have other clients who also don't share that PC.)
Along comes a woman who does not identify as a woman, but who remains legally female (has no GRC) and asks for a place, because again, this is a much needed opportunity for this individual. The organisation refuses, saying their places are limited to women and she doesn't meet their criteria of sharing a "woman gender".
She sues for discrimination on the basis of gender reassignment because the organisation allows other female people access who don't share that PC and their declared aim is to improve things for women, for whom the legal definition under equalities legislation applied to this case is "a female of any age". Which she is.
In both cases the organisation would lose, because discrimination is always unlawful unless the organisation applies one of the exceptions allowed for under the provisions of the Equality Act 2010. And those exceptions are strictly limited and there is no exception that allows an organisation to conflate two protected groups in the above manner.
(It is legal to combine two or more PC and offer a service only for disabled children or pregnant women or black men. But you cannot offer a service for disabled children for instance and then include some disabled adults or some children without disabilities and/or exclude other disabled children.)