I should clarify that I have never studied history, so please correct any stupid assumptions that I make. I read somewhere (I forget where), that Druids were forbidden from writing down their secrets. This led me to wonder what writing system and language they would have used if they were to break this rule. I assume that each tribe would have their own language, and that these languages were all related and are probably related to modern Welsh. As for what writing system, I know that the Roman alphabet has been used in Britain since the Romans settled here, but what would the British tribes have used before the Romans? I assume they must have had some kind of writing system, but I have no idea what that would be.
In short, we don't know.
We only have a tiny handful of examples of Brythonic writing, and these take the form of 'curse tablets' found at the Roman temple-bath complex to Sulis-Minerva at Bath Spa in Somerset. These are written in the Latin alphabet and engraved in the same manner as the other tablets.
Not to discourage further answers but this was also asked a few years ago here: https://www.reddit.com/r/AskHistorians/comments/2raktx/what_script_did_people_in_whats_now_england_and/