You should submit an application form BOTA to register for citizenship if you are a British national who is also a national or citizen of another nation. You should submit an application form BOS to register for citizenship if you are a British national and do not have any other nationality or citizenship.
If you are eager to know more about Registration as a British Citizen we suggest you follow some of the criteria that a British citizen must have.
citizen of the British Overseas Territories
British citizen living in abroad
British Protected Individual