United Kingdom - Passport & Nationality - British Overseas Citizen (BOC) - Upgrade (11(1)2.1.a.1971)
Where an applicant was born before 01.01.1983 and holds a British Overseas Passport (known as a BOC) AND has a grandparent born in the United Kingdom (or Ireland before 31.03.1922), then this applicant will already be a British Citizen.
When claiming through the male line (through the father or grandfather), then the birth of each subsequent generation must be legitimate (or subsequently made legitimate).