Didn't we do this one to death a whileback?

Descriptive names like the "Painted Ones" - Prytani and Picts are generally applied by outsiders. We know that recent hunter/gatherer tribes like the Inuit call themselves the Inuit but it just means The People. In other words we are US and everyone else is THEM

I reckon the early peoplle of Britain didn't havea name for Britain at all because they had no concept of the land as a whole archipelago. Each community has their territory and that was home. Names for other places would have been descriptive or personal. So there is no original name for Britain. You can consider the names used by the Romans or the Greeks or the Phoenicians or any other visitors, but I don't think you will find any name used by the inhabitants.