As an avid caver in this region I was aware of the use by Paleo/Neolithic peoples of the more open rock shelters as dwelling places, perhaps on a temporary basis. The seemingly deliberate placement of various artifacts in the "non-livible-in" swallets appears to be another example of how our ancestors propitiated the "Gods" of the unknown.
Swallet holes can certainly appear over night especially after heavy rain and can be very deep. Manor Farm swallet , near Charterhouse, opened up during the floods of 1968 into a shaft over 11M deep and would have been accessible with primitive ropes had it happened in the past. Unfortunately it is now used as a farm dump. What a legacy to leave our descendants!
I found a fascinating article online about Swallet holes on the Mendips, and their possible uses in the Neolithic. Swallet holes are natural vertical caves - they sometimes open up overnight, and are difficult and dangerous to enter. But they've been found containing bones, fine stone axes, pottery, and stones that aren't local geologically.
The Priddy Circles are in an area with a very high density of swallet holes, which date to older than the circles themselves. Is this why the circles were sited here? What connection is there between them? It's all very interesting.