The earliest known references to the islands as a group appeared in the writings of sea-farers from the ancient Greek colony of Massalia. The original records have been lost; however, later writings, e.g. Avienus's Ora maritima, that quoted from the Massaliote Periplus (6th century BC) and from Pytheas's On the Ocean (circa 325–320 BC) have survived. In the 1st century BC, Diodorus Siculus has Prettanikē nēsos, "the British Island", and Prettanoi, "the Britons". Strabo used Βρεττανική (Brettanike), and Marcian of Heraclea, in his Periplus maris exteri, used αἱ Πρεττανικαί νῆσοι (the Prettanic Isles) to refer to the islands. Historians today, though not in absolute agreement, largely agree that these Greek and Latin names were probably drawn from native Celtic-language names for the archipelago. Along these lines, the inhabitants of the islands were called the Πρεττανοί (Priteni or Pretani). The shift from the "P" of Pretannia to the "B" of Britannia by the Romans occurred during the time of Julius Caesar.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Where did the first known occurrence of the British Isle name appear in written material?
writings of sea-farers