In the OSR link, they mention this, published by the ONS on 14 April:
www.ons.gov.uk/news/statementsandletters/genderidentityincensus2021
"The census findings on gender identity are broadly consistent with other sources including previous Government Equalities Office estimates and other comparable countries’ data. The estimated number of transgender people is slightly lower in the census than in a separate 2021 NHS survey.
The question on gender identity was tested thoroughly and the whole census questionnaire was available translated into nearly 50 languages to ensure it could be properly understood. When other formulations of the gender identity question were tested – for example simply asking respondents whether they are transgender – the question was less well understood than the final formulation that was used in Census 2021."
And the OSR says they're going to do a review:
"The scope of our review will be formed around the principles within the Code of Practice for Statistics and will consider ONS’ approach to responding to questions raised by users about the results of the data. It will not consider gender identity as a concept or the decision to collect data on gender identity."
I assume those principles are important. They must be these ones?
code.statisticsauthority.gov.uk/the-code/
"The framework for the Code of Practice is based on three pillars – Trustworthiness, Quality and Value.
Each pillar contains a number of principles and detailed practices that producers should commit to when producing and releasing official statistics. On each Code principle page there is a table with guidance and resources and links to case studies.
The Code also has three cross-cutting themess_, areas of practice that don’t fit within just one pillar – collaboration, coherence, and transparency.
The Office for Statistics Regulation (OSR))_ reviews compliance with the Code. OSR also offers Code training seminars and workshops for statistics producers."
Although that states Code, it's the closest to Principles that I could find. So was the ONS question based on trustworthiness, quality and value? And produced via collaboration, coherence and transparency? That's my best guess.