I have just looked at the passport application form, and among the notes on it -
"Because birth in the United Kingdom after 31 December 1982 does not automatically confer British citizenship, in some cases you may be asked for additional information and documentation to establish national status under the provisions of the British Nationality Act 1981 which came into effect on1 January 1983"
http://www.fco.gov.uk/Files/kfile/Guidelinenotes.pdf