The NHS app doesn't show anything like a QR code which would link to a URL and allow for cross-verification
We are therefore left with an immigration official having to trust what is visible on a passenger's mobile. I imagine immigration officials will be very reluctant to touch the phones of lots of potentially infected people
What stops somebody creating a simple, but fraudulent, app (and selling it, but not via official places like Google) which gives a display of the relevant bits of text to keep immigration happy, and charging people for a download to their phone ?