According to a story from The Washington Post the other day, the police in London are searching through the tapes of over 2500 closed circuit televisions for clues to the bombing.
Sky News is running a story that says it's likely that the bombers all died during the bombing.
These cameras did nothing to prevent the bombing and they can do nothing to solve them either because there is no one left to catch. The planners planned it well out of the way of the prying lens.
So, do all of these so-called security measures do ANYTHING to prevent terror? Not likely.