The Gender Recognition Act 2004 allows for a person to change their legal gender to their preferred gender.
To change your legal gender and obtain a Gender Recognition Certificate (GRC) you will need a report from a specialist clinician recognised by the Gender Recognition Panel, in addition to a medical report from your GP.
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.
You will also be able to marry a person whose gender is opposite to your legal gender and enter into a civil partnership with someone of the same gender as your legal gender.
Your pensionable age will be the same as your legal gender, and all of your legal rights and responsibilities will be the same as your legal gender.
If you hold a GRC, it’s 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.