I would say that this vastly under counts what could be included as "hillforts". Here in Ireland there are "Raths" every couple of Kilometres, mounds upon which some sort of human activity was carried out, probably settlement. The clue is often in the Gaelic placename, for example "Rathangan", "Rathmolyan", so the Hill fort of a certain famous person lost to history. I assume there may be a similar trend in the U.K, most probably Scotland anyway.
A Hillfort or Rath or something similar would be a great terrain project, if a little impractical for play.