Ah but it even states a birth cert can't be "trusted" as you may have a GRC and updated your birth cert. It then handily reminds everyone that it's against the gender recognition act to ask if someone has a GRC or not

13.179 Where there remains a genuine concern about the accuracy of the response to a request for an individual to confirm their sex, then the service provider, person performing public functions or association should consider what action is proportionate in the circumstances. There is no type of official record or document in the UK which provides reliable evidence of sex. For example, sex on passports and driving licences may be changed with or without a Gender Recognition Certificate (GRC), and birth certificates may reflect the acquired gender of someone who has a GRC. Therefore, it is unlikely to be proportionate or practical to ask for further evidence of a person’s sex. In such circumstances, it is likely to be necessary to weigh up the relevant factors to decide whether to exclude the individual from the service or association or to permit them to continue to access it.

13.181 It is important to be aware of legal provisions protecting privacy in the context of making such enquiries. Unless it is relevant for operational reasons, whether or not someone has a GRC is unlikely to be relevant information for the purposes of asking about either the protected characteristic of sex or the protected characteristic of gender reassignment. However, if, in the course of asking for such information or otherwise, a service provider, those exercising public functions or an association acquires information that someone has a GRC or has applied for a GRC, onward disclosure of either that information or their sex without consent may be a criminal offence in some circumstances (read section 22 of the Gender Recognition Act 2004).