It's not that simple. You could have a perfectly clear description of an illegally discriminatory group.
The default position is that groups, services, association etc are open to anyone.
If there is a legitimate aim (and for associations this is quite a low bar - 'because we want to' is sufficient in most cases) you can have a 'limited membership' group.
But there are rules on the ways in which you can apply a limit. You cannot exclude by protected characteristics - the old 'no blacks, no Irish' tenancy rules are totally illegal. But you can include only certain PCs - so you can have 'only women'. (There are some extra rules for race, so 'white only' isn't allowed).
You can have multiple inclusion criteria, but if you do so the members of the included group must all meet all criteria. A group for disabled lesbians can onky admit people who are women and same sex attracted and disabled; it can't allow disabled men, same-sex attracted women without a disability, same-sex attracted men etc.
So a lesbian group (PCs: female, same sex attraction) is fine. A group for transwomen attracted to women (PCs: male, gender reassignment, opposite sex attraction) would also be fine. But you couldn't have an association that included both groups because they have different protected characteristics, and you'd be illegally discriminating against the excluded groups.
What I think you could do, if you really wanted, is have a lesbian association and a 'sister' association for TW attracted to women (and possibly also one for TW attracted to TW). The associations could then hold some joint events. Like Scouts and Guides (if both were still single sex) having a joint jamboree.