There is no such thing as a "10% law they have to give you..." - it is a suggested guideline.
10 years ago ACPO guidelines were produced that suggested the trigger for the issuance of a Fixed Penalty Notice to be when travelling at 10% + 2 mph for all limits except for 20mph, in which case 10% + 3 mph = 25 mph. Speeds significantly in excess of the limit should be dealt with by Court Summons.
http://www.acpo.polic...elines_web_v7_foi.doc
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..
As to the speedometer reading, you have that the wrong way round - it must never under-read the speed you are travelling but is allowed a maximum 10% + 2.5 mph (4 kph) over-read. e.g. if your speedo reads 30 mph the law allows your road speed to be 25-30 mph; no more, no less.