The sheer fucking stupidity of these people.
For a start, the only trans women who are women in the eyes of the law are those with a GRC. They represent a tiny portion of the whole (a few thousand at most), and even then they are not treated as women in all legislation. The Equality Act, as Sonia points out, says that single-sex spaces can legally exclude trans women.
But the vast majority of people who call themselves trans women do not have a GRC and are therefore not recognised as women in any part of the law.
Finally, even if the law said that trans women are women (which it doesn't), it still doesn't actually make them women. Everyone with a modicum of sense knows that it's not possible to change sex, and that trans women are men. Full stop.