the danger areas are ‘colony gate’ and Ash Ranges.
http://www.streetmap.co.uk/map.srf?x=491500&y=158500&z=120&sv=colony+gate&st=3&tl=Map+of+Colony+Gate,+Surrey+&searchp=ids.srf&mapp=map.srf
Colony Gate (bang in the middle of the linked map) is properly fenced off and cannot be entered (unless you can bunny hop 8 feet) In any case, don’t.
Ash ranges (to the south of the map) is open access although is often closed off by very visible red flags when live firing is taking place. There is stuff in there, and even the perimeter road is a good fitness bimble but again, when red flag is up, don’t think about it.
The rest of the military areas, usually marked at main entrances with a white signboard with all the necessary bylaws written on them are fair game. they are active exercise areas, and often have troops practicing on them, in which case I consider it courteous to stay out of their way rather than exit the area completely. Use discretion, if you encroach on fitness training runs / carrying stretchers, etc. that is less likely to cause problems than riding straight through a dug in position or an ambush!! It has been done, and the rider in question fair shat himself when the world lit up as he rode through a tripflare! Needless to say, don’t touch anything you find on there.
There is plenty on the exercise areas, less in the danger areas, for obvious reasons. A few groups ride / tailbuild on there, so take one of the likely looking side paths off the main tracks and see where it leads you!