I don’t think your description of the changes to building regulations are accurate.
They haven’t passed any regulations yet. As of 3 weeks ago, they intended to propose the new legislation in parliament, with the intent that it would come into force next year. I imagine that had been waylaid by the GE announcement.
And the change is to legally require separate men’s and women’s facilities provided there is sufficient space to do so. Previously there was no requirement and universal toilets had been becoming more popular, which is what the new changes would be intended to combat.
Under the proposed changes, there would be no requirement for unisex toilets.
The changes wouldn’t stop transwomen from using the women’s toilets or transmen from using the men’s, either.