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They're usually prisons, power stations, special forces training bases and the likes.
....and the odd upper class twit of the year's country estate.
Oddly enough, the Queen doesn't have one around Sandrigham at any time, even at Christmas when the entire clan gather for their annual tupperware party.
Most restricted areas around prisons only apply to helicopters, although Full Sutton High Security Prison has a GA airfield next door - most convenient!