It's a little known fact, that you can claim back 17.5% VAT on 'tax-free' purchases at UK airports!
Original 'street' price - 117.50
Dixons Tax Free price - 100.00
You then ask Dixons for a VAT receipt upon purchase. They will give you a receipt showing that you have paid £85.11 for the item, and £14.89 tax!
Try it next time you go and see what I mean! My last purchase where I got a VAT receipt as above was just 4 weeks ago at Stansted. Dixons TF have worked this way for years.
Obviously to claim back the tax, you need to be VAT registered and the purchase should be for business purposes only!