Well, I'm not a lawyer! With any luck some of the more knowledgeable women on here will chip in.
In theory it is still possible to exclude males from some certain situations/places (like refuges) no matter if they identify as transwomen. In law the Equality Act could be called upon to make certain posts/places exempt and thereby protect single sex provision.
But in practise, I'm not sure whether this happens.
There have been examples of rapists in women's prisons, Katie Dolatowski was placed in a woman's refuge after assaulting a child in toilets.
My understanding is that any male arrested who declares he is a transwoman will be automatically put in the female estate and assessed for risk afterwards. This was revealed during the recent Judicial review for keeping prisons single sex.
twitter.com/NoXYinXXprisons
I'm sorry, but from what I can tell although self ID is not law, but it may as well be.