There are other unions doctors can join such as the HCSA (hospital doctors only, I'm not sure if GPs have much choice).
I suppose the BMA are going to be concentrating on the industrial action, but maybe they need to make sure somebody is updating their policies in line with the law (that's the law handed down from the Supreme Court, who are actual experts on the law, and not something dreamt up by the resident doctors committee, who aren't). They've got advice on their webpage which says in the section on "Gender and personal care"
"When a person has affirmed their gender to be different than the sex they were assigned at birth, they are protected from discrimination. The sex they were assigned at birth becomes irrelevant to their current working and day-to-day lives."
and " in some instances, for the psychological safety of a patient, there may be occasions where accommodations can be made about the presenting gender of a professional who treats a patient – this would be on a case-by-case basis. For example, if a patient had been the victim of sexual abuse by a person of a particular gender, they may ask for a professional to examine them who physically presents in a different gender."
So just "presenting in a gender" is fine!
www.bma.org.uk/advice-and-support/equality-and-diversity-guidance/discrimination-guidance/managing-discrimination-from-patients-and-their-guardians-and-relatives/exceptions