Paragraph 16
A person in any of the following categories may be admitted freely to the United Kingdom on production of a United Kingdom passport issued in the United Kingdom and Islands or the Republic of Ireland prior to 1 January 1973, unless his passport has been endorsed to show that he was subject to immigration control:
(i) a British Dependent Territories citizen;
(ii) a British National (Overseas);
(iii) a British Overseas citizen;
(iv) a British protected person;
(v) a British subject by virtue of Section 30(a) of the British Nationality Act 1981, (who, immediately before the commencement of the 1981 Act would have been a British subject not possessing citizenship of the United Kingdom and Colonies or the citizenship of any other Commonwealth country or territory).
Paragraph 17
British Overseas citizens who hold United Kingdom passports wherever issued and who satisfy the Immigration Officer that they have, since 1 March 1968, been given indefinite leave to enter or remain in the United Kingdom may be given indefinite leave to enter.