I think there are three types of shops/services airside and different rules apply.
Bars/Resturants who sell goods for consumption on-site. Prices for these goods are not free of excise duty or VAT. They do not claim to be 'duty free'.
Shops selling Booze, nicotine fixes and Perfumes. These goods, if exported from the EU, are offered as 'duty free'. If so, it is required to prove you are leaving the EU. Showing your boarding card is mandatory. It may be possible to purchase these goods for EU travel but the taxes are added on. It is not necessary to show your boarding card. These shops claim to be 'duty free'
Other shops selling other goods. If they can prove the purchaser is leaving the EU the retailer is not subject to the VAT. To prove this the retailer needs to scan your boarding card. However there is no obligation for the passenger to allow this. These shops do not claim to be duty free.
There are hybrids of two and three above, where they claim to be duty free and do sell duty free goods to passengers leaving the EU. Their claim to be duty free is partially correct, but not all goods sold there are duty free.
It is the second group that is ripping us off. Knowingly selling goods at prices inclusive of VAT, when they are not required to remit the tax to the UK authorities if the passenger is exported the goods from the EU.
And then there are the airlines where they maintain a single price for onboard sales for all flights, inclusive of UK VAT, which is charged to all passengers including those on non-EU flights. Often the goods in their boutiques are referred to in the announcement as duty free (and quaintly as 'duty-frees) Now I don't buy on board, primarily because the value for money is nonexistant. But those that do are being royally ripped off.