In the CTEEA committee meeting today, Joan McAlpine pointed out that 3% of the sample of 2000 members of the public said they would consider not answering a sex question if it were based on self-identity.
Nat Records for Scotland have made a big play about the possibility that a fraction of the trans population might not answer an actual sex question, but they don't seem to have been too worried that women might object, and they don't seem to have tested for women's likely non-response.
Would Mumsnetters consider boycotting the census if the sex question is a de facto gender one such as is currently proposed by both NRS and ONS?