Even if items are being sold at the same prices to all comers and advertised as VAT or duty-free, VAT and duty still apply to items which are not being sold for export. The retailer still has to account for the status of every item sold and pay the appropriate amount to HMG where tax is due. Obviously, this means that their effective cost of sales differs depending on the destination, so they want to get it right to maximise their profits, not to mention avoiding a compliance audit from HMRC.