Moralia 419e = On the Disuse of Oracles 18

Demetrius said that of the islands around Britain many are remote and desolate, and that some of them are named after gods and heroes. He himself had sailed to the nearest of these remote isles on the instructions of the emperor for the purpose of enquiry and observation – an island which had only a few inhabitants, though these were all holy men and held inviolate by the Britons.