The Ministry of Defence own and lease a lot of land in the UK and much of it is used for military training exercises. In many places these ranges and training areas are also open to the public and have public rights of way across them.
These areas are indicated on the Ordnance Survey 1:25,000 maps with red triangles for 'live firing' ranges and open red triangles where public access may be allowed. You can view the 1:25,000 maps on Streetmap by clicking on the + sign under Zoom Control.
You can use a very useful link called MapIt! a so you can check any coordinates/postcode on Streetmap.
Range with 'Open Access' - Open red triangles
Range with live firing - solid red triangles