I know The Telegraph reported at the start of this year that Scotland Yard uses CCTV footage in most of its murder investigations and 7 out of 10 murder cases are solved that way. On the other hand, the source of the data is Scotland Yard, so an independent study is probably called for.

The interesting question to me is whether the public would welcome that much surveillance if the govt had been honest enough to admit that the object wasn't to prevent terrorism/crime but to catch people after the fact.