That was my read-aloud book in 3rd grade, I want to go back and see just how much Welsh Lloyd Alexander put in it. I remember lots of words starting with 'gwy' anyhow.
Yes, the name Prydain in The Chronicles of Prydain by Lloyd Alexander (i am reading that series now, btw), is another form of the name "Britain". Also of the same root ar the names Brittany, Breton, and Brythonic (of the Celtic languages, Breton, Cornish and Welsh ar Brythonic; while Scottish Gaelic, Irish, and Manx ar Goidelic). Indeed, the original Brits wer not English-speakers but "Celtic Britons", who spoke Proto-Brythonic, a Celtic language. Germanic peoples invaded in the 5th century AD, giving rise to English. Proto-Brythonic gave rise to Welsh, Cornish, and Breton (some Celtic Brits fled to Brittany to escape the Germanic invaders, hence Breton).
The Pretani were the original inhabitants of Britain, this is the same name as the Cruitani (Cruithne) the original inhabitants of Ireland . Thats the P and Q celtics... as in: head. pen. ceann 4. pedwar. ceithir 5. pump. coic.
note that germanic and latin are also P and Q languages 5. phunph. cinq 4. phi’er.. quatre