Changing your gender legally (by obtaining a GRC) is a personal decision. This is not required by gender identity clinics, employers or other institutions.
Once you obtain a GRC it is illegal for people who know that you once had a different gender (having obtained that information in an official capacity) to pass that information on, unless for very specific purposes such as the detection of a crime or for relevant medical purposes. Even then, it is good practice to seek your consent.
Once you have a GRC you will be able to obtain a replacement birth certificate with your preferred (and now new legal) gender on it. Your pensionable age will be the same as your legal gender, and all of your legal rights and responsibilities will correspond to your legally updated gender.
If you have further questions please contact:
Email the Gender Recognition Panel at GRPenquiries@hmcts.gsi.gov.uk
Telephone: 0300 1234 503