ONS have released the final guidance on the Sex question in the census today.
Link here
Haven't read through it all yet, but below is the guidance it looks like they are planning to use. They specifically mention GRC as a 'legal document' you can use to classify your sex, as well as passport (which doesn't require GRC to change).
What is your sex?
Why we ask this question
Your answer is key to understanding trends in the population. It also helps your local community by allowing charities, organisations, and local and central government to understand what services people might need.
This information will be used for equality monitoring between groups of people of different sexes in your local area. Your answer also helps public bodies to identify discrimination or social exclusion based on sex, and work to stop it from happening.
The sex question has been asked since 1801.
This question is vital for understanding population growth and equality monitoring. Please select either “Female” or “Male”.
If you are considering how to answer, use the sex recorded on one of your legal documents such as a birth certificate, Gender Recognition Certificate, or passport.
If you are aged 16 years or over, there is a later voluntary question on gender identity. This asks if the gender you identify with is different from your sex registered at birth. If it is different, you can then record your gender identity.