@DadJoke
I don't think you are being consistent.
not legitimate or proportionate, or based on any existing case law around loos
It would seem very legitimate and proportional to gender critical women. Facilities are available to the trans woman but he wants to use the single sex space. That space is single sex. It could end up being the case law.
The no assumptions about how many transgender people is one of your own making. There are many transgender people in my hypothetical company. All but one are happy using the facilities in complance with the three space policy.
The hypothetical trans woman has outed themselves many times in general conversation with other staff. It's a really big important part of their identity, and they make sure everyone knows it. It's impossible not to know they are a trans woman.
People trying to enforce the policy includes HR, who enforce all company policies on all staff equally.
Your alternative? forms a central part of my hypthetical question already. They are already well catered for, they just don't like not being able to use the female single sex space.
It is discrimination. That's the only thing we agree on. My question is : Is it lawful discrimination under the Equalities act exemptions that allows for discrimination where it is necessary and proportionate?