Found this on another forum;-This is a quote from Honda UK (sounds about right)
"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation 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 per cent of the true speed plus 4 km/h(2.5mph). In production, however, a slightly different tolerance of 5 per cent plus 10 km/h(6.0mph) is applied. The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests."
If the Road Vehicles (Construction and Use) Regulations 1986 act allows use of speedometers that meet this EU regulation, UK specific regulations can't be any tighter.