Many years ago, 10% +2mph was the effective limit - people were only prosecuted for exceeding this threshold. IIRC at pretty much the same time that the previous government brought in hypothecation (the police receiving the revenue from the fixed penalties), some bright spark realised that the guidelines allowed for enforcement at the threshold - so driving at 10% +2 over the limit is asking for trouble, whereas 10% +1mph is generally fine, as long as you don't slip up, or the camera over-read by 1mph, etc.