WR1992 have been confirmed as sex-based. The problem is with employers' discretion to allow in some men exceptionally (as SSE rules don't apply).
Upton and Peggie both have protected beliefs (though I would argue that his are non-WORIADS to the extent they require unconsenting women to grant him access).
His beliefs have state endorsement, but the effect must be limited by Article 9.
So the employer must accommodate both, requiring neither of them to share intimate space with men.
This is not onerous for the employer. It should be easy to find out how many gender non-believers they have, and allocate strictly single-sex provision accordingly, without breaching anyone's confidentiality.
And they must do that, to avoid a harassment finding like Fife suffered.