I used to shoot on the grounds of an RAF/Naval base and just asked the CO for permission. But i was a serving senior nco at the time.
I now have 1500 acres to use.
And it depends on what the MOD land is used for. If its land owned by the MOD and just sitting dormant you may gain permission. If it is being used for training purposes then you have no chance, same goes if there are buildings on the land.