They usually have a sign saying NATS and a warning regarding any interference with the equipment.
Some are right next to main roads, a few that come to mind are :
Davidstow,Cornwall or is it Devon. Anyway adjacent the airfield on the main road.
Winstone ( also kwnow as Birdlip in the past I recall) next to the main road between Cirencester and Gloucester.
One thats difficult to miss is Clee Hill near Ledbury, Herefordshire due to the golf balls stuck on the highest bit of ground in the area.
But these are in my part of the world so may be of no help at all.
I dont think security is an issue even in these times due to the fact that the general location can be found in many books relating to airband listening ao scanners in general.