The HM Revenue & Customs web-site has information on UK Visitors VAT Refunds. The guides tell you how to get a tax refund on goods bought in the UK from shops that offer Tax Free Shopping. More detailed information is given in the document VAT Notice 704/1 Tax Free Shopping in the UK which is available online or from shops operating the Scheme or local VAT Business Advice Centres. Retailers may make a small administration charge but most of the time no charge is made.
Some shippers and purchasers may also be able to assist you in claiming VAT back from purchases.