http://www.legislation.gov.uk/id/ukpga/1981/61

PART IIIBritish Overseas Citizenship

26Citizens of U.K. and Colonies who are to become British Overseas citizens at commencement

Any person who was a citizen of the United Kingdom and Colonies immediately before commencement and who does not at commencement become either a British citizen or a British Dependent Territories citizen shall at commencement become a British Overseas citizen.