There have always - until now - been separate prisons for men and women
because:
A completely separate wing of the men's prisons should be set aside for transwomen, even those with GRC
That way they are safe from men and not made to feel uncomfortable by them, without risking the safety & comfort of women prisoners
Women prisoners can't choose not to be there and they are there 24/7,
so that is quite different to the situation outside prison,
where some of us agree that a transwoman with a GRC could access access single sex loos, changing rooms etc