They are neither trained nor qualified to be able to spot a forgery or a phone hack
This really doesn't matter.
How many people do you actually think are going to forge or hack the NHS app to show a false vaccination?
I'd wager less than 10 in a 55,000 strong crowd.
If a steward falls for it then it doesn't really matter, especially as the other 54990 will be vaccinated anyway.
The crime just isn't worth the effort needed.