Paragraph 18
In the case of an applicant—
who claims to be a British citizen, but not a citizen of a British Overseas Territory;
who does not currently hold a valid United Kingdom passport describing the applicant as a British citizen;
who was not born or adopted in the United Kingdom;
who was born before 1st January 1983; and
whose father was a British citizen at the time of the applicant’s birth,
the applicant’s passport on which the applicant first entered the United Kingdom and either:
the applicant’s birth certificate in a form which contains details of the parents, if issued by a British consulate or High Commission, or
the other documents specified in sub-paragraph (2).
The other documents specified for the purposes of sub-paragraph (1) are –
the applicant’s birth certificate not being a certificate issued by a British consulate or High Commission, in a form which contains details of the parents of the applicant, in so far as they are included on the relevant register;
the applicant’s father’s birth certificate or their certificate of registration or naturalisation; and
the applicant’s parents’ marriage certificate .