Margins of error are 'guidelines' not law apparently
//Manufacturing a perfect speedometer and display with zero% error would cost an inordinate amount. The ACPO recognise this and apply 10% plus 2mph. This is not law, these are guidelines. It is entirely possible for a constabulary to set speed traps with a zero margin of error.
This table shows the guidelines THE POLICE use for letting drivers exceed the speed limit, however, some cops and some cameras will catch you for going just 1 mph over the limit:
Speed Limit Of the Road Fixed Penalty Usually issued at speeds above: Summons usually issued at speeds above:
20 mph 25 mph (10% + 3mph) 35 mph
30 mph 35 mph (10% + 2mph) 50 mph
40 mph 46 mph (10% + 2mph) 66 mph
50 mph 57 mph (10% + 2mph) 76 mph
60 mph 68 mph (10% + 2mph) 86 mph
70 mph 79 mph (10% + 2mph) 96 mph
//
http://www.motoringfirst.co.uk/MotorOffenceBible/TheLawInTheory.php
a useful site on motoring law and penalties