Thanks for your response! I'm going to have to take a look into that 7th century writing when I have the chance. I was of course aware that by the later dark ages Celtic languages were written down (which, by the way, together with English pose an enigma to me: why are these "vulgar" languages written down as opposed to the rest of Europe, where Latin remains the only written language for centuries)
A quick question though: Is it possible that Celtic languages being written down in the 7th century is a new development?
Because my argument was that they werent being written down during the migration period, i.e. 5th and 6th century. I assumed that for the entire Roman period, celtic languages were almost never written down, but I would love to stand corrected.
Nevertheless, my premise still stands: as a literary language, Latin reigned supreme in the Roman west. Therefore, Celtic languages would not have been able to hold their ground as elite languages such as Latin did in Gallia or Hispania. It simply had less 'cultural authority' to withstand the regime change. A Frank in Gaul would be confronted by a venerable and prestigious Latin. Nothing personal, but I doubt that Brythonic could demand the same awe from the Anglo-Saxons. Just like Punic or Berber or Gaulish or Thracian, etc. Never were able to match the prestige of Latin. So of course Celtic was established, and maybe it was written down, but it didn't have the prestige Latin had, nor its associated cultural package of Romanitas.