Asked ONS a Q about this via the standard enquiry contact details. Below is an extract from the ONS response.
‘Census 2021 will continue to ask a binary question, with the response options female and male; this approach is unchanged since 1801.
There is also a new voluntary question on gender identity later in the questionnaire for people aged 16 and over, providing data for which there is significant user need at a national and local level.
Our design for Census 2021 will ensure that the data we collect from the sex question in Census 2021 will be of the highest quality.
Achieving the highest possible response rates both nationally and locally for the census as a whole and for each question is vitally important. This is key to ensuring we can produce the best possible statistics in our census outputs, fulfilling the needs of census data users and helping to inform decisions that will affect us all in the years ahead.
We will be providing online guidance to assist respondents in answering all questions in the census questionnaire. This includes guidance on the question on sex, although, as with previous censuses, the vast majority of people will not need help, or to reference the guidance at all, to answer this question.
We have worked closely with organisations and individuals from representative groups over the last few years on the guidance for the questions in the census, including the guidance for the question on sex. Through this engagement and our testing and research we have concluded that directing people who are considering how to answer the question on sex to use one of their legal documents, such as birth certificate, gender recognition certificate or passport, was clear and inclusive.
By referring to ‘legal documents’ the guidance makes clear we are referring to government issued documents. This is not self-identification, which was evaluated as part of a range of options but not taken forward. Both the question itself and the online guidance clearly indicate that there is a separate gender identity question for those aged 16 and over.
The final wording of this guidance and further details of the work done on it were published on 12 February in an article on the Office for National Statistics (ONS) website, available here.
www.ons.gov.uk/census/censustransformationprogramme/questiondevelopment/genderidentity/census2021finalguidanceforthequestionwhatisyoursex