committees.parliament.uk/oralevidence/12639/pdf
Q24 answer hasn't aged well, given the OSR is examining this data:
"the good thing about the recent census is we have started to get some figures, and it would appear from the recent number crunching of the census that there is one trans person in every 704 in the population. It, perhaps, is still an underestimate because people are still reticent, perhaps, of declaring themselves to the state in terms of numbers. That gives us, perhaps, a handle on how rare what we are talking about is. Broadly one in 1,000, one in 700."
Anyway, Q32:
"It is not actually a practical difficulty. I do not have a GRC. I transitioned just before the Theresa May period, and was looking forward, like the first same sex marriage people, to being the first person on the steps to get an English GRC under the new system and I waited for that, but I practise under my female name. I pay my taxes under my old gender. There is a practical example of why the IT issue we heard about earlier is nonsense because I have one tax number, and I pay my taxes perfectly happily, but I appear to be two different genders if you look in different places for me in the tax records."