I get the sense that some people don't understand that the census is all about gathering data for use by the government, councils etc. An individual return is confidential for a century after the census date. It's not an opportunity to tell the world 'I identify as ...' in any way that is linked to your own name. If the questions aren't answered correctly, the data becomes a lot less useful.
Made up example: Joe is male, and his birth certificate says M. When Joe is 45, he comes out as trans and henceforth is called Jo. Jo doesn't get a GRC, as can't see the point. Jo has hormone treatment, but no sexual reassignment surgery, as Jo has some heart problems that make it risky to have major surgery for anything except a life-threatening emergency.
Jo should click M on the census. There's an optional question on gender identity. If Jo feels comfortable answering that, that's the place to inform the government about Jo's trans status.
The data from the census helps the NHS plan what services might be needed in future. If lots and lots of Jos ticked F, not M, the NHS may conclude that there are more middle-aged and older women around than they thought and fewer men of that age. So they up the number of clinics and specialist HCPs dealing with breast cancer, ovarian cancer, menopause etc etc. Fewer services for older males, e.g. dealing with prostate cancer.
This is a problem for Jo, who isn't at risk of breast or ovarian cancer. If Jo gets a sex-specific cancer in old age, it's likely to be prostate cancer.