Visitors from non-European Union countries can reclaim the sales tax (VAT) on purchases from a number of British retailers.
To claim, first purchase goods from any merchant displaying the Tax Free Shopping sign. You'll be given a tax refund form by the retailer – keep it along with your receipt.
Then when you get to the airport, there is a Customs post box located just after security (on the back wall). Post your receipts and accompanying documentation and this will be stamped and forwarded for processing. Your tax refund will be paid at a later date.