The problem is, the Brits still can't decide on the definition of a "Celt". If you go there to study the subject, as I have, you will be alternately amused and appalled at their "Anglo-Provincialism". Many British scholars still think the Celts are those "smaller and darker" people who live in Cornwall and Wales. (It turns out they are just the offspring of shipwrecked sailors from the Med area who hit the rocks off Penzance, over thousands of years. There has long been heavy ship traffic between the regions.)
Then, hop over the little pond to Ireland and see what they have to say about Celts. It's entirely different. To the man on the street "Celtic" means "Catholic", as in the Holy Celtic Church. They are seen differently at the Universities too.
But it's only when you back off and take a continent size, or better a global historical view that you see who the Celts really are. Click on my FR Profile, below for their REAL history.