Britannia: British goddess. There is some evidence that the ancient Celtic residents of the island of Britain saw their land as a goddess of Sovereinty, like Eiru of Ireland. This controversial view has detractors as well as supporters. A very early Roman coin (161 C.E.) from Britain shows a female figure mounted on a globe and bearing a shield and spear; she has been interpreted as Britannia, a name also used of England during its imperial period in the 18th and 19th centuries. The name may derive from the tribal name Pritain. (Celtic Mythology and Folklore -Patricia Monoghan)

Check out Pritain. You will find some early Welsh poetry using the term. Interestingly, in the Welsh language, the letter 'p' can mutate to 'b'. I shall not go into that, please.

Myles Dillon and Nora Chadwick say, in "The Celtic Realms", The oldest name under which Classical writers commonly refer to the British Isles is "The Pretanic Islands" (4th century BC). The name is probably Celtic... used in Welsh texts for the island as a whole...used in the form Prydyn to refer to the people north of the Antonine Wall, the Picts.

Britannia: "a poetic name for Great Britain" (dictionary). If I remember rightly, the earliest peoples of the island called it Briton. Or, perhaps the Romans named it Briton. My memory slips here. John Davies, historian, says "Brutus and his companions (1170 BC) were the first of the Britons and the ancestors of the Welsh. I wonder about that. You've challenged me to do some digging there. Anyway, you can find much about the Britons in Mr. Davies' book, "A History of Wales".

That is probably more than you ever wanted to know and maybe doesn't even answer your question. If not, I am sorry. Someone else can do better. Hazel