Not quite correct, TWR.
Speed limits in the UK are laid down in Schedule 6 of the Road Traffic Act 1984:
http://www.legislatio...ga/1984/27/schedule/6
You will note that all the limits (with the exception of a couple at the bottom relating to tractors) are in multiples of 10mph. The one relating to large HGVs (60/50/40 as mentioned) is at 5.2 (b).
The confusion arises because an EU directive is in force which relates to speed regulation equipment fitted to such vehicles. This states that the equipment must be set to 90kmh (56mph). When it was first issued the UK obtained an exemption to this (we were allowed to govern the vehicles to 60mph) but this expired in 2006:
http://www.dft.gov.uk...imiterlegislation.htm
(See third paragraph and .pdf table)
So vehicles fitted with governors are governed to 56mph, but their speed limit (on motorways in the UK) is 60mph. So whilst there may be an offence of failing to fit or properly calibrate the speed limiter, the offence of speeding is not committed unless 60mph is exceeded.
Yet another example of unnecessary confusion caused by EU regulations!