Dai: We keep a token Welshman in the office for such details, but the budget doesn't run to an etymologist. Welsh (as distinctly Welsh) goes back to the 7th century but Logres/LLoegr apparently doesn't appear before the 12th. Which doesn't mean it wasn't there, of course, but it still leaves it open whether it is an old Celtic word or a new word altogether, possibly adopted from Anglo-Saxon. Given that it describes a state that didn't exist as such in Roman or pre-Roman Britain, I would expect it to be a new word.