The British Nationality Act 1981 contains three provisions which allow certain adult foreign citizens to register as a British citizen if they are of good character and:
they were born to a British mother between 7 February 1961 and 1 January 1983;
they renounced citizenship of the United Kingdom and colonies before 1 January 1983; or
they renounced British citizenship on or after 1 January 1983.
In other cases, foreign citizens who wish to acquire British citizenship need to apply for naturalisation provided they meet certain residence and other requirements set out in the 1981 Act. Those who are married to a British citizen must be of good character and have a satisfactory knowledge of the English language and life in the United Kingdom. They must also have lived lawfully in the United Kingdom, without long absences, for at least three years and be free from immigration conditions by the time they apply.
Further information about the requirements for registration and naturalisation is available on the Home Office website at www.bia.homeoffice.gov.uk/sitecontent/documents/britishcitizenship/information leaflets.