I'm no lawyer and certainly don't want to start a legal argument, but doesn't the discretion refer to admin? As I said I'm fortunate in that I haven't need to test the application of the guidelines (and with most of us I suspect that it is just good fortune!)
The guidance was updated in April 2010, prior to that one of the footnotes said:
3 ACPO bottom level threshold – ACPO set a limit below which speed offences should not be prosecuted so as to allow for equipment inaccuracy, speedometer inaccuracy and human error interpreting a speed indication by dial. Prosecutions below this level should be undertaken by way of careless driving or other offence, not speed offence alone.
Still puzzled in Stoke!
The answer is that no amount of advice from the ACPO alters the law - it's 30 in a 30 limit. Anything above that is discretionary and police forces are independent so it's up to them whether they take notice of the advice or not.
