According to Directgov.co.uk, if you are entering the United Kingdom from outside the European Union and carrying more than 10,000 euros, you need to declare to HM Revenue & Customs by filling form C9011. Cash includes bank notes, coins and travellers cheques.

You can get the form from the airport and should fill it before being asked by the Custom Officers from the UK Border Agency. Otherwise, you will be fined for not declaring.

You also need to declare the money if you leave the UK to anywhere outside the EU and take more than 10,000 euros.