As a non-European visitor visiting Europe you will pay VAT on most things you buy. The Retail Export Scheme allows you to claim a VAT Refund on most goods you intend to export from the European Community. You must export them by the last day of the third month following that in which the goods were purchased.
Should the retailer be part of the Retail Export Scheme they will give you a VAT Refunding Document. This should be presented to Customs along with the goods when you finally leave the EC. Without a Customs Stamp the form will be invalid.
With the form properly validated you have a number of options. You can post it back to the retailer, return it to a VAT company (available if the retailer has outsourced this service), or should there be a cash refund office at the departing airport, the refund can be made in cash.
Travelex acts as an agent for a number of Refunding Agents, Global Refund and Premier being the major ones. Should their forms be presented at various Travelex locations at either Heathrow or Gatwick Airport in the UK then they can be exchanged for cash instantly. There is no extra charge for this service.