Tristan da Cunha is one of the few UK Overseas Territories not to have its own currency. The island does, however, have a Commemorative Coins Ordinance which enables the Governor (with the prior approval of the British Secretary of State for Foreign & Commonwealth Affairs) to "cause coins to be made and issued to commemorate notable events of importance to Tristan". Coins made and issued under the provisions of this law are legal tender on the island; they are not really likely to be used as such though, because their collectible and metal value far exceed their face value.
Some of these coins are in the Half Sovereign 22-carat gold format which mirrors the specifications of the British Half Sovereign, which (starting with the pre-decimal sovereign) has been issued in this form since 1817. The full sovereign has a nominal face value of £1 (one pound), so the half sovereign is formally denominated as £0.50 (or 50p - fifty pence).
Tristan da Cunha sovereigns are issued at prices with a premium over their melt value which is much higher than that of bullion coins, so technically speaking they should be considered Non-Circulating Legal Tender (NCLT) and not bullion. | 
