Back in the day I used to buy a fair amount of clothing at LHR T4, when it was BA's terminal, and also T5. They had some brands there that I don't have at home, and I was often flying through at clearance sale times (yes, airport shops have clearance sales!). I bought Bally shoes, quite a few things from Reiss and Escada, some Paul Smith, some Harrod's, etc. The prices were quite reasonable for those brands, considering the duty-free aspect as well as the sales.