If we accept the SC judgement, Upton should not have been in the female changing room
I'm obviously stupid, because I just don't see this as a given. I really really want to, but I can't.
It's certainly true for single sex services, because it's only by sex that you have permission to segregate services, and to admit men to a women's toilet vitiates adherence to the gateway condition that segregation by sex (which must mean biological sex) is necessary as a PMOALA.
But - we are not in a public service. We are in a workplace, and there's nothing in the section of the EA2010 that applies to workplaces that says discriminating by sex in a workplace environment is acceptable as a PMOALA. There are no gateway conditions to meet, no equivalent of the Schedule 3 exceptions to the "no discrimination on any PC" rule. You cannot discriminate by sex in a workplace - period - at least not in what facilities you offer to your employees.
When we're talking about a service, it's held that even if there's no obvious disadvantage (there are both men's and women's toilets, so nobody is at a detriment, and discrimination requires detriment) it's still discrimination because someone could argue they are at a detriment, but that's ok because of the schedule 3 exceptions. So far so good.
In a workplace, it clearly isn't inferrable that separate sex bathrooms are a detriment to either sex otherwise separate sex bathrooms in the workplace would be de facto unlawful - remember there are no permissive exceptions for sex discrimination for PMOALA in a workplace - there is nothing similar to section 29, Schedule 3.
So - if there isn't a gateway condition to providing separate sex toilets in a workplace - there's no possible argument that separating any other way than by sex vitiates the gateway condition. The reasoning why the only legal separation is by biological sex just doesn't work like it does for services.
That means that while separating by biological sex in the provision of toilets, changing facilities etc. is certainly legal, it may not be the only legal separation.
There are good arguments working outside the EA2010, like the workplace regulations, a requirement for consistency across bathroom provision, and probably some others. But I still can't see how anything that the SC said in FWS is relevant.
Somebody - please, knock down this reasoning.