Well that's the problem with the PC of gender reassignment, isn't it? It's the only PC which requires other people to believe in a belief system. It actually impinges on the lawful rights of others.
If someone's gay, it doesn't require anything from me. If someone's a Muslim, doesn't impact me. But if someone believes they're a woman when I know they're a man, their belief impacts on my lawful rights, if they come into the women's changing room.
I'm allowed to say god doesn't exist, though other people believe he does. Am I allowed to say 'you're a man' to Dr Upton? How can Sandie exercise her lawful rights, both the right to a single sex space that is single sex (PC of sex). and the right to believe that sex is fixed binary and immutable (PC of religion or belief) without harassing Dr Upton?