I'm surprised HMRC has not done an audit of sales and seen issues...I regularly scan the wrong boarding card (I just extract one from my bag and don't check the details. Managed to use a Lufthansa BUD - MUC one at T5 a couple of weeks ago! I then got creative / bored and tried one of the self-service checkouts, and scanned a barcode on a receipt from somewhere else....yep, worked fine!

Clearly the scanners are not taking genuine information from the boarding card to decide VAT/not VAT....