Absolute nonsense - I don't suggest you try it.
Sorry Niall, even the 35 in a 30 myth is not law! The speed limits are the law so 30.0001 is breaking it. Aside from all sorts of procedural issues that can affect 'guilt', the Association of Chief Police Officers has GUIDELINES which are generally interpreted as 10% plus 3 (so 80 in a 70) but there are many other factors.
Don't forget manufacturers speedo standards allow for over-reading and although modern cars are more accurate than the cable driven ones of old, speeods typically still overread 5%+......assuming the driver hasn't changed the wheel/tyre size! So at 80 indicated on a motorway, a standard car is well below ACPO guidelines.....but they are only guidelines, not law.
If anything there is talk of reducing the speed limits to 50-55 to save fuel, but I suspect that's just a rumour.....but look at how many 60 mph A roads are now 50.....
The Speed Awareness Course is not available everywhere - only certain Police Authorities offer it and they differ in terms of eligibility. As the SAc generates revenue, I reckon we may well see more of these....£100+ vs £60 FP....go figure!