A British citizen, of full age and full capacity, may make a declaration of renunciation of citizenship under Section 12 of the British Nationality Act 1981.
Applications should be made using Form RN1. The current fee for such an application is £229.
Further guidance can be found at Chapter 19 of the Nationality Instructions, a copy of which I will place in the Library and which is available on the UK Border Agency website at http://www.ukba.homeoffice.gov. uk/sitecontent/documents/policyandlaw/nationalityinstructions/nichapter19/.