ArkRoyal - Weird that, coz my aircrew one has worked fine at LGW whenever I've wanted to visit the 'duty free'.
Aside - are you aware that the prices offered in the 'Duty Free' are typically 17.5% less than the RRP ( this is so that it looks to Joe Public like it's still duty free ).
However the best bit is that when you make a purchse ( e.g. laptop ) the retailer will give you a receipt - complete with a VAT number and the VAT applied - so if you, or your company, are registered for VAT you can also claim the 17.5% VAT back.... ergo your goods are being purchased for 35% less than RRP, and they still have all the usuall warrantees that you'd associate with a major retailer.