If you are travelling to or from a country outside the European Union you must declare any cash of over 10,000 euros or more (or the equivalent in other currencies) to HM Revenue & Customs. Find out what you need to do to declare cash when entering or leaving the UK.
http://www.direct.gov.uk/en/TravelAndTransport/Foreigntravel/BringinggoodsorcashintotheUK/DG_173289