The box of tricks with the meter and switches, which were actually relays, is BT line tester which were installed to help the engineers diagnose faults at communication critical sites. You were in the 'frame' room where all the BT kit was housed.
Probably dates back to the 70s. In the telephone exchanges there would be rows of these where the testing clerks would work when You used to ring 150 to report a fault on your line.
Great explore this one :-)
Ah.. I remember 150 and that other number you could call that would immediately make your phone ring.. and then there was nobody there when answered.
Great for winding other people up in your house, or whatever house you were in.
BT must have left their gear behind then. I wish the cops had left more. It was slim pickings for such a large place.