Perhaps it was limited in our area, if you look at the map the Pool farm cist where the foot one was found is very near to Priddy circles, plus the Ashen hill and Nine barrows groups. Signifiying a place of great sanctity - a gathering place. Same really goes for Knowlton as well with its henges. Therefore it maybe has to be be a central place or maybe a particular mark of a certain type/clan of people.(northern relatives) Similarly Wales has a few cupmarks distributed round, mostly to the north, and probably by the sea, this was presumably carrying on the irish tradition of marking stones, again its sporadic not like up North which has thousands of cupmarked stones......