I'd say that was an impact crater because of the debris lip thrown up around the edge. May have been a stray shell from Worbarrow gunnery range, or a smaller bomb dropped during WWII. It was common for German pilots to drop their load and run for home if a fighter stumbled across them.
Mind you, Worlebury Hillfort near Weston Super Mare still has the remains of 18 or so storage pits/ grain silos inside the fort.