The HM Revenue & Customs web-site has information on UK Visitors VAT Refunds. The guides tell you how to get a tax refund (currently 17.5%) on goods bought in the UK from shops operating the VAT Retail Export Scheme. More detailed information is given in the leaflet VAT Notice 704/1 VAT refunds for travellers departing from the European Community which is available to download online or from shops operating the Scheme or local VAT Business Advice Centres. The forms (VAT 704, provided by the participating shop/retailer) allow the retailers to make a small administration charge but most of the time no charge is made.
Alternatively, there are commercial companies offering more universal tax refund schemes. Global Refund is the world's leading tax free shopping service and shops displaying the sign on the right subscribe to their services. With these companies, an administration charge is made and often smaller purchases are not worth the hassle for the resulting small refund. Check out the Global Refund web-site at www.globalrefund.com.
Some shippers and purchasers may also be able to assist you in claiming VAT back from purchases.