European Commission
Gives details of customs regulations in the EU for tourists.
http://europa.eu.int/comm/publications/archives/booklets/eu_glance/08/txt_en.htm

Global Refund
Tax free shopping service.
http://www.globalrefund.co.uk/

HM Revenue & Customs
Official guidance on customs allowances, banned and restricted goods, and tax refunds.
http://www.hmrc.gov.uk/
http://www.hmce.gov.uk/

TNT Magazine.com
Features an unofficial but detailed and freely downloadable guide UK visas.
http://www.activemag.co.uk/tntguides/
http://www.tntmagazine.com/travelzone/

UK CITES
British regulations on the import or export of endangered plants and animals explained.
http://www.ukcites.gov.uk/

UK Visas
An official guide to the country's entry regulations.
http://www.ukvisas.gov.uk/
http://www.homeoffice.gov.uk/

Visit Britain
Contains information about passports, visas, immigration and, of course, customs regulations.
http://www.visitbritain.com/in/fact_files/planning/immigration.htm

Visit Britain
Offers an overview of customs rules and regulations within the EU.
http://www.visitbritain.com/beforeyougo/byg-5-1.htm