The excerpt from the GRA that Snappity posted is legit and they are correct on the interpretation point, although it's a fairly nuanced point and it's particularly important to bear in mind that history cannot be rewritten and the GRA only changes the interpretation of the documents going forward from the date that the GRC is issued.
This point is just plain silly though:
Sex becomes that shown on the new birth certificate "for all purposes".
There are no exemptions to that. Sex is absolute.
After saying that upon a GRC being issued the legal sex changes, the Act goes on immediately to say that this "does not affect things done, or events occurring, before the certificate is issued" and that this "is subject to provision made by this Act or any other enactment or any subordinate legislation." I think any reasonable person would regard those as big exceptions. The explanatory notes included with the Act even refer to exceptions!!
"9. Subsection (3) means that the general proposition is subject to exceptions made by the remainder of the Act and, for the future, by any other enactment or subordinate legislation."
I think Snappity is getting too focussed on the wording tbh. Yes, the Act might say that a person's sex changes but it is, after all, a piece of legislation rather than magic and therefore in the real world it just means that people with a GRC should be treated as their new gender/sex. In the same way, companies have corporate personalities under the law but I hope even Snappity wouldn't believe that eg their bank is really a person.
So if you take s19 of the GRA for example which says that a GRC "does not affect the descent of any peerage or dignity or title of honour," that means that the person with a GRC is effectively still treated as their birth sex for the purposes of the succession rules, and it is therefore an exception to the general position that people with GRCs are treated as their new gender/sex. It is an exception regardless of how it is phrased. It doesn't have to literally say that the person with a GRC is still their original sex for succession purposes, that is it's effect.
I'm just going to reiterate that the legal position is just the legal position. It does not change material reality. Parliament could pass a law saying that grass is now a rather fetching shade of fuchsia if they wanted to, but the grass would still be green.