'@lechiffre55 I am not a lawyer.
As things stand, I think excluding the single trans woman from the women's loos is not lawful as it's not legitimate or proportionate, or based on any existing case law around loos.
First, you can make no assumptions about how many transgender employees you have, and the ruling in a company of that size can make no assumptions about it.
It would, at the very least, involve outing any transgender employees (and there may well be more than one). It's also entirely uenforceable unless you (a) insist on all employees telling the employer whether they are transgender (unlawful), inspecting everyone's genitals (illegal) or taking an enforced gene test (also unlawful).
It will also result in any GNC employees or guests being potentially subject to harrasment by people trying to enforce the policy. It is absolutely not proportionate.
The alternative is that gender critical people who object to transgender people using the loos which match their gender have the option of using the loo which matches their sex or the unisex one, which I think is a reasonable accomodation.
I'm not sure if it's direct or indirect discrimination, but it's certainly discrimination.
It would make an excellent test case.