In meetings between health ministers and the nhs back in the 2000s, the made an active descision to allow the sex field to be either sex or gender depending on the patients wishes.
They could've introduced a new gender field and retained the sex field but didnt.
Thats why when they had initiatives around single sex wards, they knew they ment gender, but still advertised it as sex. And its why they have to warn people changing their marker about sex specific screening.
Whats odd about all if this is that it come as a surprised to the NHS, ministers and the media.