That NHS data collection recommendation says nothing about "wrong body", though. They're trying to elicit accurate information about something potentially medically relevant (assigned biological sex). Their concern is that if you ask a trans person simply "male or female" you'll get an answer that either reflects the gender someone is living in, or the sex on their birth certificate. The former is useless if you're trying to assess the risk of chromosomal disorders, the latter if you're trying to work out how to politely address them.