We could tell you but then we'd have to shoot you
It is a NATS facility exactly as it says on the sign. The pictures on the net show pretty standard VHF comm antennae on one tower. Not so standard on the other towers but as NATS offsets the costs of running its infrastructure by leasing space to other users of the radio spectrum it is not unusual to find that on NATS sites.
The fortification has more to do with the state of our society and the price of copper than anything else.
There's a cone of silence above communication facilities so you'll normally find them offset from airways rather than beneath them.