I finally found the report in question.

http://www.google.co.uk/url?sa=t&source=web&ct=res&cd=3&url=http%3A%2F%2Fwww.urbaneye.net%2Fresults%2Fue_wp6.pdf&ei=TLmTSpeYGM2OjAeg-OnnDQ&usg=AFQjCNF_9yjlzfqr3FLnhUaQOiWLHHXM2g&sig2=pCh8-yftX5AAfgYR-L7ysA

The section that all these numbers come from is:



(sorry about the image, the PDF doesn't allow copying)

They basically surveyed 269 shops/offices on a busy London high street.

Even by their figures, of the 500,000 CCTV cameras they think London has, 85% of them are in private businesses. Elsewhere in the report they reveal that the London Borough of Wandsworth has just 210 open street cameras, which equates to 1 open street government operated camera per 1338 residents.
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Remind me to change my signature to something more interesting someday