Some rather dry but nevertheless interesting statistics for you: According to the Equality and Human Rights Commission (EHRC), as many as 1% of people may have gender “issues” of some description or another.
The evidence comes in the form of a “technical note” on conducting surveys from the EHRC. (PDF Link) When asking just over 10,000 people various questions about their gender, 0.4% did not identify as binary (I.e. neither male nor female) and 1% had had thoughts about (Or taken actions towards) gender reassignment.
Of course, this new evidence hasn’t stopped the Daily Mail having a pop at previous, lower estimates, labelling them “preposterous” purely because relatively few people actually get as far as dealing with the NHS (…perhaps because it’s so difficult to do, and the likes of the Mail create a negative social stigma around it…) or getting a GRC (…because they’re really not as much help as they should be, thanks to the Equality Act!)
But I am inclined to agree with the Daily Mail (I know, sorry, it happens occasionally) about excessive collection of data. It’s rarely statistically significant for Trans folk unless you’re dealing with a sizeable number of people and can be intrusive. (Data that one person in a group of 100 is Trans is enough to out someone)
Oh, and of course if you collect data, you can accidentally leak it. (It’s not clear in that case if information on gender reassignment was also leaked)