The EA is part of my job. The bar for exclusion is high. You must have a good reason why a trans person cannot do a particular task or activity. The EA does not allow blanket exclusion.
I assume Cath does some sort of HR or employment related work, because it is correct that employment law would prevent you from stopping a trans person doing a particular task. Eg you can't refuse to let a trans person answer the phone just because you're squeamish over your customers' reaction to hearing "Hello my name's Arabella how can I help you" in a bass voice.
That does not mean that Arabella here is legally a female if she doesn't yet have a GRC. She remains a male with the protected characteristic of gender reassignment. For most purposes (answering the phone is not a sex specific task!) Arabella should be treated "as a woman" with her preferred pronouns respected etc. If the company arranges a women's lunch that is purely social in nature, there would be no legitimate aim served in excluding her from it.
However, if the team goes on a weekend away involving overnight communal accommodation, Arabella can be excluded from the women's accommodation, because she is still legally male, and it is legal to exclude someone with the protected characteristic of gender reassignment in these circumstances.
There are two current legal opinions on what happens if she acquires a GRC thereby becoming legally female.
Opinion 1 (which I share): Arabella is legally female. Those with the protected characteristic of g.r. can be excluded. But those with the protected characteristic of female sex cannot, otherwise it makes a mockery of the exemptions. Therefore Arabella must be entitled to join the women in the communal accommodation.
Opinion 2 (which I do not share): Arabella is legally female. But because she has a history of g.r. she can still be excluded even if she is female.
[Option 2 is what some transsexuals are afraid of: that the EA will be interpreted in this way, thereby throwing them straight under a bus, but never mind them eh, they're only 1% of the transgender movement these days (that was sarcasm Cath, which is different to irony). I also think that this cannot be what Parliament intended, partly because the GRA creates a legal sex "for all purposes" and partly because if we conclude that female people with a protected characteristic of g.r. can be excluded, we mean that women who look a bit butch can be excluded too, as they have the p.c. of perceived g.r.]
And this is where the concerns over reform to the GRA arise, because at the moment there are relatively few Arabellas, they've always been in and around women's space, and that is partly because the GRC process is one which has a degree of integrity. If we replace that with a simple stat dec, and Opinion 1 is correct, then it opens up the protected category of female to a very large number of ill-intentioned, or part-timing, males. If Opinion 2 is correct then sex exemptions are retained but trans women who do have a GRC are excluded.
The govt needs to be clear over which they are inviting us to do.
I have a background in this area. (I don't expect Cath to believe that, but others might find it useful.)