That was done by looking at the tapes afterwards. But hiw.many crimes were actually prevented by the cameras? Do they provably act as a deterrent? In how many cases did the cameras result in help arriving on time?
Only MI5/GCHQ/OSCT can answer your questions with actual numbers.
(EDIT)
There are several studies on effectiveness of camera surveillance on crime prevention.The UK unlike other nations actually has a lot of material here.
I haven't followed up. Are these numbers published? If not, I would tend to assume that they are too embarrassing to show. But despite my bias, I'm actually curious.
