It's not clear at all whether a GRC makes a difference or not to our rights under the EA.
I've read some good analyses from proper lawyers who say that tw with the PC of gender reassignment, but who do not have a grc, must not be discriminated against compared with any other male people. So the everyday single sex exceptions (Schedule 3, part 7, paras 26-27), that allow us to have separate male and female facilities in the first place, are sufficient to exclude these male people. And that therefore the sole purpose of para 28 is to also exclude male people with a GRC who are legally female.
This exception applies to the specific circumstances where harassment is a likely consequence of not using it:
(a) the provision of separate services for persons of each sex;
(b) the provision of separate services differently for persons of each sex;
(c) the provision of a service only to persons of one sex.
And harassment does not require a comparator anyway, so the whole thing is as clear as mud.
We're dealing with two poorly drafted pieces of law that interact with each other and there is no relevant case law. If EHRC were doing the job we all pay them for then they'd have brought cases to clarify this area of the EA, ideally on behalf of women.