a person is entitled to be treated as the acquired gender ‘for all [legal] purposes’.......subject to [any other] provision made in ..... any [other] legislation ........ In short – the [law] expressly allows those in possession of a GRC to be treated for some purposes as their birth and not acquired sex.
Many thanks to @KnottyAuty and @selffellatingouroborosofhate for providing the useful link and quote about this, which I've condensed even further, above. I'm embarrassed to admit I had been struggling with how the GRA and EA could be logically consistent with each other, but this explains it.
There's a problem though, isn't there? The GRA renders the medical history and GRC status of a trans person (and therefore, of any person) private personal information, and instructs officials to keep it confidential. That's an obstacle to treating those in possession of a GRC ... for some purposes as their birth and not acquired sex.
An official with explicit investigatory powers (say, the criminal justice system or a sporting regulator) won't have a problem, but everyone else does. What are the options?
Birth Certificate? Nope.
Genital inspections? Ewww. Nope.
Buccal swabs? To use a changing room? Really?
Shouting 'he's a man'? Not going to go well. Transphobia! 😱
The EHRC gives the example of a rape survivors' group which can legally exclude transwomen in order to spare members the trauma of encountering a biological male. How will they do it in practice?
And in the unlikely event of a perfectly passing legally female AMAB flying under the radar in such a group, how will the women feel if they find out?
Concealing your sex just shouldn't be allowed.