Altimeter, really.
An experienced pilot does not need an altimeter to distinguish between 187 and 500 feet agl. And if this loop had been correctly flown , correct entry speeds, height and power setting, full, and had not used max pitch rate on the way up ensuring gate height or more, he could have tightened up the looping radius to ensure correct radius and recovery height on the way down AND would have had a safety margin of 500 feet to eliminate the possibility of ground contact.
It's as simple as that. What we may never knowing why it was so badly flown, and if AAIB can't pinpoint the causes no amount of speculation on Prune wil help!
( Never flown the Hunter but a lot of experience on the JP including some low level aeros)