Why didn't they use CB Radio's?
The schoolmaster and the London group were the only ones I really noticed using HAM/CB radios in this series. Rather than waste time on the 4pm call at the mast why didn't they just collect together a bunch of HAM equipment? They could have at the very least acquired the equipment from a bunch of taxi cabs with the central base. With this they could have covered a really nice area 30sq miles or so for local comms. With decent homebase setup they could have managed a huge radius. They didn't even think about taking the gear from the school on the second trip. Greg could have collected kit together on his travels and dropped them off at each community or at least advised them to find the equipment themselves and join a certain frequency to communicate on and they could pass messages up and down the country with ease.
I suppose the telephone mast comms were at least secure in the sense that no one else had thought of it but this could have easily been solved through means of certain key phrases and codes or something mundane that wouldn't alert suspicion.