Liverpool street?

It wasn't a darkened room at all though it may seem so in the video. The lighting in the cabins was no worse than in an average house.

As for basic LUL signalling the principles are quite straightforward although the basic building blocks are usually put together in a different way at every site and something odd or otherwise unexpected is often thrown in to add to the confusion. That said I agree that sitting in a room playing trains is not quite as much fun as working on the equipment especially under failure conditions in a difficult environment. You can't beat the adrenalin rush of trying to work on say 10s points (10B these days) in the tunnel at Heathrow with trains whizzing round the loop one behind the other following the failure! Mind you I think the most exciting thing I ever did on the Central was track rewiring on the resignalling changeover of Holborn, Chancery Lane and St.Paul's way back in 1978 or 1979. We did it with everything live which made it rather electrifying to say the least.

As for darkened rooms, they are of course the control rooms in engineering hours so that adds an interesting dimension to remote site testing. It was an advantage to know the signalling at the site under test!