This “10% + 2” allowance is mythical, factor, in that one can be prosecuted for exceeding the speed limit by any amount, however small. However, the Association of Chief Police Officers (ACPO) has issued guidance to police and safety partnerships (who look after the cameras) to help them decide whether to issue a fixed penalty (FP) or a court summons. The guidance for FP action does indeed follow the 10% + 2 formula and action would not normally be taken at speeds below this level. (Indeed I believe most cameras are set so as not to activate below this level). The main reason for this is to avoid large numbers of frivolous challenges involving calibration of equipment or judgement errors in the case of police following a vehicle and recording their speed manually.
For interest the ACPO guidance on the upper limit for FP eligibility is as follows:
20mph: 34mph
30mph: 49mph
40mph: 65mph
50mph: 75mph
60mph: 85mph
70mph: 95mph
At speeds above these levels a fixed penalty would not normally be offered and the driver would receive a summons to attend court. However, this is not set in stone and other circumstances might prevail which may mean those limits are reduced or increased. Additionally, of course, if the driver had nine or more “active” penalty points on his licence a FP would not be offered in any circumstances, whatever the speed, and he would have to attend court to be considered for disqualification under the “totting up” rules.