Hi,right here we go!!
Home office guidelines are 10% +2mph,that is the rule of thumb,if,worse case scenario,you are being prosecuted you have to receive the N.I.P(Notice of Intended Prosecution) within 14 days of the date of the offence --assuming you are the registered keeper.
Whilst I appreciate some Forces operate a zero tolerance on these matters I hope the above is of some assistance-
If you need any further info pse contact me.