Apple Watch Series 8 Blood Oxygen fails for 73-year-old

Apple Watch frefuses to take Blood Oxygen Measurements for a 73 year old

I've just given my wife an apple watch (series 8). We're in the UK.

The watch is bound to her iPhone SE 2020, and her details are set up in Apple Health on the phone.

The phone is running IOS 26.6 (as is the watch), so unfortunately it is up to date.

The Watch refuses to take blood oxygen measurements, despite her age being given in Apple Health as 73.

It just immediately reports unsuccessful measurement -- worked fine for me before we reset it and set it up for her on her phone/account.

Does this mean she needs to go through the whole "Verify you're 18+" process on her phone in order to use this?

She does not have a passport, driving licence, nor credit card, so that's not possible. She does have a debit card, but that apparently doesn't work, and her Scottish Entitlement Card (Old Age Bus Pass) also doesn't seem to work (even after submitting the front and back images). She's not happy.