How high

[FoxhallGH]

Here’s some information to be found: Flying drones or model aircraft recreationally

OK - that is just as vague as we've seen here in that it tends to imply vertical distances above ground only (fun video though!).
I think @Peter mavic air - you'll need to do some local research and enquiries to find out about this horizontal separation issue. Write to your CAA and pose the 500 ft cliff question ... If you fly over the edge, are you in immediate violation of the code?