Good day everyone. I'm working on a piece of fiction set in Britain during the iron age. I know very little about linguistics or Celtic languages, but I gather that linguists hypothesize a common language spoken in the British Isles during or before this time which may already have begun to diverge into the Neo Brittonic languages (Welsh, Cornish, Cumbric, Breton, maybe Pictish).
For this story, I want to construct believable names for places and characters. These may be early forms of surviving names, or invented names derived from literal meanings. I want these names to be somewhat credible, meaning they're derived from their earliest traceable forms in related languages. Obviously I need some expert help with this, whether from scholars or texts (or more likely both). Does anyone have any ideas as to first steps here? I'm much obliged for your time.