"If they are not henges, what are they?"

Jodie Lewis puts forward the argument that Priddy Circles are an intermediary form between causewayed enclosures and henges, therefore neolithic, first phase of Stonehenge had the similar internal bank and ditch. Tratman excavated the 1st circle and found turve walls and stone walls with the soil from the ditch being piled on top. It was also timber revetted with hurdling, which is another puzzle,
but she cites this as a parallel to Blackshouse Burn in Lanarkshire;-

http://www.rcahms.gov.uk/pls/portal/newcanmore.details_gis?inumlink=47640

Priddy circles had a ritual function, could be that the timbers were similar to timber circles, also they restricted the viewing to the interior, and of course access to the bank, therefore this strict only "most important people in the centre" is underlined by the ditch keeping others out - but that of course is only an interpretation....