There are two different issues being debated here.
EC Community Directive 75/443(97/39) and ECE Regulation 39 lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10% of the true speed plus 4 km/h (or 2.4mph), and that the indicated speed must never be less than the true speed.
For the past 7 years the ACPO (Association of Chief Police Officers of England, Wales and Northern Ireland) Guidelines for Speeding Enforcement have recommended, other than in the most exceptional circumstances, the issue of fixed penalty notices and summonses by a Police Officer is likely to be the minimum appropriate enforcement action for speeds based on the formula Speed Limit + 10% + 2 mph. However The Police Service now uses technology that enables it to prove that an offence has been committed as soon as a driver exceeds the relevant speed limit by a very small margin. Motorists will therefore be at risk of prosecution immediately they exceed any legal speed limit.
http://www.acpo.police.uk/asp/policies/Data/sp eed_enforcement_guidelines_web_v7_foi.doc