Gentlemen, you are misunderstanding the law on the issue. It is an offence to exceed the speed limit. Period. ACPO guidelines that you are referring to (10% +2 mph allowance) are just it, guidelines. Not all police forces have signed up to them and even those who have are not strictly obliged to follow them. So, yes, you can be done for doing 71 mph on a motorway. Of course, as a matter of practice, this is extremely unlikely, but a story of someone prosecuted for doing 32 in a 30 zone is entirely believable.