I'm not sure about this - I've seen the adverts for the book (allegedly) written by an ex-traffic cop explaining the various loopholes, etc, in the system.
One of the lines to draw you into buying the book is something along the lines of ''why you should never give your documents to the police when stopped even if you have them on you'' - was never sure why.
Someone will clarify, but under the Road Traffic Act, technically you should have certain documents with you when driving, but the police allow you the 7 days on a producer instead - not sure what they could do if you had correctly registered, insured, taxed, and MOT'd a car if you were stopped for speeding for example?