My vote goes to: Westbury sub Mendip
I am not sure if it is still open, but when I last went there in a MS-880B around 1990, it was built on an old railway line, with cinder and an embankment that fell away either side. A few years later I know someone who managed to deviate on landing down the embankment, writing off the aircraft.
Westbury-sub-Mendip - UK Airfield Guide
I have to add, that not enough PPLs experience flying into airstrips, but it requires more preparation, research and revision on the performance data. Sadly I suspect quite a few clubs in this country discourage strip flying.