If you are travelling to the EU or one of the other countries included, don’t be caught out.
Any NHS covid pass downloaded before 1st November will not work outside the UK despite being valid, in theory, for 30 days. You risk being refused to access to hotels, bars and restaurants.
If you pull up your pass directly from the NHS app each time, you will be fine. This problem only occurs if you have printed off your pass or saved it in your smartphone wallet.
This is, reportedly, because the QR code generated in the NHS app was adjusted on 1st November. This was done to move it to the EU standard and so readable when scanned outside the UK.
You can verify this by looking at gov.uk here and scrolling down to “Using the NHS COVID Pass for international travel”.