This question keeps coming up and obviously may change in the future. But the question isn't is it allowed but why do so many women's groups not bother, not care or dont know?
Quote from the Equalities Office:
Providers of women-only services can continue to provide services in a different way, or even not provide services to trans individuals, provided it is objectively justified on a case-by-case basis.
The same can be said about toilets, changing rooms or single-sex activities. Providers may exclude trans people from facilities of the sex they identify with, providing it is a proportionate means of meeting a legitimate aim.
www.independent.co.uk/news/uk/politics/transgender-people-no-right-single-sex-spaces-government-penny-mordaunt-toilets-changing-rooms-a8414771.html
See also guidance from FPFW www.fairplayforwomen.com/changing_rooms/