Thira

It's not really fair to blame the Brits for renaming this Greek island.

According to Wikipedia, its "classical" name is (or was) Thera. The name Santorini is derived from Santa Irini (Saint Irene) – dedicatee of an Early Christian church on the island. This name was given to the island in the 13th century by the Latin Empire (a feudal Crusader state, founded by the leaders of the Fourth Crusade on lands captured from the Byzantine Empire).

Before that, the island was known as either Kallistē (meaning 'the most beautiful one'), Strongylē ('the circular one'), or Thēra. Wikipedia goes on: "The name Thera was revived in the nineteenth century as the official name of the island and its main city, but the colloquial name Santorini is still in popular use."

Elsewhere, Wikipedia gives the current official name as Thira.