Don't confuse guidelines with law! Speed limits are just that, limits which are legally enforceable. As Mike has said, you can be prosecuted for 1 mph over.
As for speedo readings, the law states that they may over-read by up to 10%, but must not under-read, so manufacturers build in a little bit of over-read just to be on the safe side. My DS3 over-reads by 2mph throughout the speed range, as verified by GPS.
A good rule for learner drivers: ignore anything your mates say!
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