All excellent points, but I was making only a single, not particularly sophisticated, point about the different categories of protected characteristic.
A PP suggested that letting Upton into the women's CR would be discrimination against the rest of the men, who are excluded.
Whilst there are lots of arguments against (it is disadvantage, not exclusion, that counts, and the men are not disadvantaged, because they have their own CR: the LAPA that allows sex-segregation in spite of sex discrimination law is arguably not fatally undermined by allowing in a small number of transwomen: if letting in TW is in itself sex discrimination (because women are more disadvantaged than men are by mixed-sex provision) it could still be rendered legal by calling on a LAPA related to trans welfare, and so forth).....
.....there's a single overriding reason the suggestion doesn't work. Which is that illegal discrimination against trans people (someone with the PC of GR is unjustly disadvantaged compared with someone without that PC, and of the same legal sex) can exist, but the reverse legal concept doesn't exist.
You asked under what circumstances would [anyone] be favouring someone with the protected characteristic of gender assignment, over someone who doesn't have it?
I don't know. But if I want to pay my trans employees twice what I pay everyone else, then the EA won't stop me. That is all.
(It's a really trivial point, and I'm kind of regretting mentioning it now!)