I want to know the answer to the following question:
When did the "P" sound of Pretannia change to the "B" sound of Britannia?
All the background I'm provided with is that "In the 1st century BC, Diodorus Siculus has Prettanikē nēsos, "the British Island", and Prettanoi, "the Britons".". Is that enough to answer the question?
Answer: no