I'm curious - are non-active MOD bases illegal, or is it just a telling off?
I guess it then depends upon how determined the person making the is to see you punished. At one active MOD establishment it was MOD Plod's habit to inconvenience the poachers they picked up by detaining them until morning and then releasing them on the opposite site of the establishment to their access point and vehicles (it was a big place). Even trespassers at AWE were not always prosecuted, but nowadays I expect that they make use of the caution to make sure that you end up with a record.
Bottom line ... nowhere is a telling off the only available option.