The GRA says of a man with a GRC "the person’s sex becomes that of a woman".
It does, but section 9 (3) says 'subject to provision made by this Act or any other enactment or any subordinate legislation.'
TRAs don't shout about that subsection very much.
www.legislation.gov.uk/ukpga/2004/7/section/9
If I've understood your posts correctly, you see the EA single sex exceptions as the normal everyday exceptions and then the gender reassignment exceptions as something extra-exceptional on top, with a higher bar?
That's not how they read in the EA itself. Gender reassignment exceptions are just there alongside sex exceptions with no additional bar and no distinction between those with a GRC and those without one. For example, in Schedule 3 (services and public functions), part 7 para 28 says you can treat people with the PC of gender reassignment differently because of anything done in relation to providing single sex or separate sex services.
www.legislation.gov.uk/ukpga/2010/15/schedule/3/part/7/crossheading/gender-reassignment
Paragraphs 26 (separate sex), 27 (single sex) and 28 (gender reassignment exception) all have the same threshold - proportionate means of achieving a legitimate aim.
I can't think of any circumstances where paras 26 or 27 would be applicable but 28 would not be applicable.
Where we come unstuck is with the EHRC Statutory Codes of Practice, which fit under the definition of 'any subordinate legislation' because they have been passed by parliament in a thing called a statutory instrument.
www.legislation.gov.uk/uksi/2011/857/made
SIs are what the government reckon the law says. They can be challenged by judicial review but it's very expensive and stressful.
Despite their recent 'clarification' EHRC Statutory Code still says that excluding any individual tw from a female only space can only be done on a case-by-case basis, regardless of what policies an organisation has in place. It also says that tw with a GRC should only be excluded from female only spaces in extra special truly exceptional circumstances which probably shouldn't ever happen.
But none of that is in the EA itself. EHRC made it up with the extreme help of the usual dodgy trans lobby groups. Following a recent consultation on the enforcement of the EA and the role of the EHRC, the Women & Equalities Committee asked the EHRC to write new, much clearer Statutory Code and lay it before parliament but EHRC refused. Because there is no case law (which is part of EHRC's remit).