I wonder if this calls for a really radical rewrite of the statutory and non-statutory guidance around SSEs.
For the moment, we seem to have the ironic result, that those women-only entities that are permitted under sex-discrimination law, but not because of reasons of safety, fairness and privacy (public board quotas, shortlists, prizes, associations and clubs), can exclude TW just because they are men.
But those women-only entities that are permitted under sex-discrimination law because of reasons of safety, fairness and privacy (toilets and changing rooms, prisons, refuges, sports teams) can only justify the exclusion of TW by reference to a proportionate means to a legitimate end, which is more onerous.
The guidance has an adverse repercussive effect by implying that the default is trans-inclusion.
So the guidance on trans-inclusion should be rescinded in favour of a 'sex means sex' interpretation, particularly if the entity is governed by other legislation that mandates sex-segregation (eg regulations about prisons, school toilets, and workplace changing rooms).
Whilst entities that are permitted by the Act, but not mandated by other legislation, could be trans-inclusion optional.
Disclaimer: haven't read the judgment, and probably should before speculating further.