Answer: daily double! (Just kidding.)
Why not make cameras that output only encrypted video which may be stored by agencies and decrypted only when people holding the right keys come together during a trial or active police investigation? That would be strictly better for privacy than what we have now - which is total surveillance.
If a criminal chooses to not commit a crime (or commits it elsewhere) because the risk/reward balance is changed, then it prevented the crime from happening (there), and made that location safer.
So there is no doubt that the cameras make places safer. Even fake cameras can make a location safer. The question is whether the cost to personal integrity makes it worth it. Most importantly: who answers that question, and who ensures the cameras aren't used for bad purposes? That's probably a smaller problem in London than in Beijing.