• About 262,000 people aged 16 and over said their gender identity was different from their sex registered at birth. This was 0.5% of those who answered the question. [ons.gov.uk], [ons.gov.uk]
• The census question was voluntary, and around 45.7 million people (94%) answered it. [ons.gov.uk]
• Among younger adults aged 16 to 24, about 1.0% identified as transgender, which was the highest proportion of any age group. [ons.gov.uk]

A few important caveats:

• These figures cover England and Wales only, not the whole UK. [ons.gov.uk]
• The Office for National Statistics has noted uncertainty around these estimates and now classifies them as“official statistics in development” rather than fully accredited official statistics. [ons.gov.uk], [osr.statis…ity.gov.uk]
• If the same proportion (roughly 0.5%) were applied to the UK’s current population, it would imply around 330,000 to 350,000 people across the UK, but that is only a rough extrapolation, not an official UK-wide count.

So the short answer is: the most widely cited official figure is about 262,000 transgender people (0.5% of adults) in England and Wales based on the 2021 Census. [ons.gov.uk], [ons.gov.uk]