ACPO have a suggestion that to avoid disputes where motorists (or their sharp lawyers) split hairs over their actual speed vs a recorded speed, that a threshold of 10% + 2mph will eliminate such disputes. Then of course the offender is given a suspended sentence. It's no surprise that speeding and all of the negative results is prevalent among some groups of drivers.The "x + 10% rule" (sometimes x + 10% +3, or similar variations).
Interestingly, the higher threshold does not get applied to 20mph at all.