United Kingdom - Passport & Nationality - British Citizenship by Double Descent by Discretion (4L BOTC 12(5) PGF)
This route to British citizenship relies on a paternal grandfather's birth in a British Overseas Territory. If this is coupled with a parent's marriage before 1 January 1949 and their own birth in the period 1 January 1949 to 31 December 1982, it gives the candidate the ability to register - at discretion - as a British citizen.
The territories that make up this group of British Overseas Territories are: Anguilla, Bermuda, British Virgin Islands, Cayman Islands, Falkland Islands, Gibraltar, Indian Ocean Territory (Chagos), Montserrat, Pitcairn Islands, South Georgie and South Sandwich Islands, St Helena, Ascension, Gough and Tristan da Cunha, and the Turks and Caicos.