What do you mean, there are still Celts in Scotland and Ireland, and most "English" people are in fact at least parts Celt, there are very few Roman bloodlines left in Britain. Did you perhaps mean Gauls?
And in answer to what would offend a Celt, the best thing you could possibly do is kill him, behead him, and then throw his head into a ditch and take the Torque, as a Roman "hero" did in some famous battle in CisAlpine Gaul, although I don't remember the exact battle.
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