Just bumping a couple of polls about the English-Welsh-Northern-Irish Census that is taking place today!
Results, combining the Twitter and Mastodon answers and minus the "see results" responses.
About 20% of 296 respondents said they either "spoiled" the question or left if blank.
There were only 20 intersex respondents, but about 30% of them spoiled it or left it blank.
Link to Google Sheet showing calculations: https://docs.google.com/spreadsheets/d/1-qLgteuaZ7Eooq-RQCzA_4EY_s1TxgiPqdxYBLCq0vc/edit
(I suspect that there must be a fair few people who entered a binary sex that is different from their legal sex, and there was no answer option for them, so perhaps a lot of them said they "spoiled" it?)
We are a Mastodon instance for LGBT+ and allies! ☺