I go through Heathrow often enough that this pisses me off:
To the anger of many travellers, some airport concessions have been reclaiming taxes on purchases for consumption outside the European Union (EU) rather than passing the saving onto the shopper. The wheeze is simple. When you line up with your sandwich, suncream and bottled water, checkout operators ask to scan your boarding pass. If it is for travel within the EU then the VAT, or sales tax, goes to the government. If it is for travel outside the EU then it should be tax free, but the shop charges you the same price and pockets the difference—20% of the retail price.
The practice is legal but many think it downright dishonest. Travellers who ask why they need to produce boarding passes for items as innocuous as chewing gum are often told that it is for "airport security". Unsurprisingly, never, in Gulliver’s experience, have they said that it is actually so they can pocket the tax differential. The practice has prompted a backlash. David Gauke, a treasury minister, said the tax-relief measures were designed to be passed onto the consumer, not pocketed by the retailer.
I usually don't buy much in the cavernous post-security shopping mall, but the next time I do—potentially three weeks from Sunday—I'm going to be more vigilant.