What is the location of the only British Overseas Territory to have used the Euro as a unit of currency?

This answer describes the situation as it stands in 2021. The British Overseas Territories (also known as BOT or the United Kingdom Overseas Territories) are, in effect, some of the left-over bits of empire. They consist of fourteen territories all with a constitutional and historical link with the United Kingdom but are not part of the UK. BOTS can be found in the Pacific, the South Atlantic, the Indian Ocean, the Mediterranean, the Caribbean, Antarctica. Now to the answer options:

The Channel Islands, consisting of seven inhabited islands in the English Channel, are not part of BOT but are two Crown dependencies: the Bailiwick of Jersey (the largest of the seven islands); and the Bailiwick of Guernsey (covering the other six islands). As currency they use the Pound sterling or the Jersey/Guernsey pound.

Northern Ireland, located on the island of Ireland is not a BOT either. It is part of the United Kingdom and uses the Pound Sterling.

Gibraltar, located, along with Spain and Portugal, on the Iberian Peninsula, is currently a BOT but uses the Gibraltar pound for currency.

The island of Cyprus currently contains a BOT: the Sovereign Base Areas of Akrotiri and Dhekelia. This BOT was never part of the European Union, but it is the only BOT that uses or has used the Euro as official currency, having previously had the Cypriot pound as currency until 1 January 2008.