This is long but I’m interested to know what pp think
EHRC guidance
2.2.7 A trans woman goes to the office of a local support group and makes enquiries with the receptionist about the group counselling sessions they offer. Based on the needs of its service users, the group provides different sessions that are single-sex or mixed-sex. The receptionist reasonably thinks that the trans woman is a biological male and, as there are some other people waiting in the office, asks her to come into a side room to get more details about the support she is looking for. When they are in private, the receptionist explains the different group sessions that are offered and asks the trans woman what her birth sex is. When she confirms her birth sex, the receptionist provides her with the details of the mixed-sex groups she could attend.
2.2.8 If there is genuine concern about the accuracy of the response to a question about birth sex, then a birth certificate could be requested. For the vast majority of individuals, this will be an accurate statement of their birth sex. However, it should be noted that a birth certificate may not be a definitive indication of birth sex. If a person has a Gender Recognition Certificate (GRC) they may have obtained an amended birth certificate in their acquired gender. In the unlikely event that it is decided that further enquiries are needed, such as confirmation as to whether a person has a GRC, then any additional requests should be made in a proportionate way which is discreet and sensitive.
2.2.9 It is important to be aware of legal provisions protecting privacy in the context of making such enquiries. If, in the course of these enquiries 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 biological sex without consent may be a criminal offence in some circumstances (read section 22 of the Gender Recognition Act 2004).