It's legal to exclude transwomen, irrespective of GRC status, from certain women-only things, provided it doesn't have an egregiously discriminatory effect (eg, they end up with no thing at all).
Institutions don't do it, because Stonewall persuaded them that trans-inclusion would fulfill a proportionate and legitimate aim of non gender reassignment discrimination.
We say, this leads to sex-discrimination, but there's no case law yet.
Avoiding sex-discrimination would be a proportionate and legitimate aim justifying trans-exclusion.
(Takes chicken across river, goes back for corn...)
I did see what looked like an error in the article, will go back and take a look...