Further reading and resources
Applying for a UK Gender Recognition Certificate
This government website has all the details. The GRA and the GRC process continue to be problematic for some people and both have major shortcomings that need to be addressed. We at Trans Legal Project support calls for self-ID for transgender people. However, in the current climate, if you feel that you are able to apply for a GRC we encourage you to do so as it will enhance your legal protection.
Trans Safety Network
The Good Law Project (active supporters of trans rights)
Press for Change
PFC was a seminal group that worked for trans rights from the 1990s. It was instrumental in getting the GRA passed. The site doesn't seem to be maintained any longer (though their Twitter account is still alive). Nonetheless, there's still lots of background material here and links to a lot of case law, etc.