I was perusing the ONS definitions of sex and gender here:
www.ons.gov.uk/economy/environmentalaccounts/articles/whatisthedifferencebetweensexandgender/2019-02-21
It has been updated recently. I don't know what the previous version said but I spotted at least two bits that alarmed me.
Firstly, it uses the phrase 'assigned at birth' in the section about sex.
Secondly, there is some waffle about how people become boys/girls/men/women through learned behaviour. So fuck knows what definition they are using for 'woman' in their stats.