https://www.legislation.gov.uk/uksi/1986/948/made
(2) Any person who is a British Dependent Territories citizen by virtue (wholly or partly) of his having a connection with Hong Kong and who, but for his having a connection with Hong Kong, would not be such a citizen shall be entitled, before 1st July 1997 (or before the end of 1997 if born in that year before that date), to be registered as a British National (Overseas) and to hold or be included in a passport appropriate to that status.