Yep ... the 10%+2mph leeway was always touted as the accepted threshold for getting booked ... seems a minority of forces have broken ranks with this for one reason or another ..... one of the things I dislike about the proliferating 20mph limits is that I know I spend far too much time monitoring my speedo and therefore correspondingly less time monitoring the road where my eyes should be.
K
One of the advantages of modern cars is that as one passes the gateway signs one can check one's speed and hit the Limit button, then you can focus on avoiding the pedestrians or whatever.