The British Indian Ocean Territory is a UK Overseas Territory. It is administered from London and there is no British diplomatic or consular representation there.
The British Indian Ocean Territory (BIOT) is not a tourist destination. Access is restricted and a permit is required in advance of travel. There are no commercial flights and mooring permits are only issued to yachts in exceptional circumstances. Access to Diego Garcia is only allowed for those on pre-authorised official duty. See Entry Requirements.
Although there’s no recent history of terrorism in the British Indian Ocean Territory, attacks can’t be ruled out. See Terrorism.
Source Article from https://www.gov.uk/foreign-travel-advice/british-indian-ocean-territory