Remember that the overwhelming majority of these cameras in London are privately owned and operated by shopkeepers and homeowners, it's not a massive state-operated surveillance network. They usually have extremely poor image quality and are hooked up to fixed local tape/hard drive recording rather than networked.

With that said, there are still a lot of state-operated cameras. Mostly to make money from traffic infraction fines, curiously that's one area where governments don't ever seem to cut police funding :-)