UK Covid Pass "expires" during holiday in Spain

A bit more digging later: https://www.nhs.uk/conditions/coronavirus-covid-19/get-digital-covid-pass/

How long a digital NHS COVID Pass is valid for A digital NHS COVID Pass is valid for 30 days. The 30-day period refreshes and starts again each time you log in.

If you have an NHS COVID Pass for travel based on a positive PCR test, the 30-day period will refresh for up to 180 days after you took the test.

If you use the NHS App for your NHS COVID Pass, it may ask you to confirm your phone number when you log in. If you do not confirm it, your NHS COVID Pass will be valid for only 72 hours.

If you download your digital NHS COVID Pass as a PDF it will be valid for 30 days. Always check the expiry date before using it.

So curiously, the act of signing into the NHS app, makes the vaccine and booster work a little longer! ;-)

It may seem strange that you can update your pass by the simple act of logging in and requesting a new one. But I think the motivation is that the pass should be available from your phone, whether or not you have access to the internet at the time that you need it. So it is valid for thirty days, after which you must check in with Covid Central to keep your status up to date.