Don't different areas set their own limits, here in Worc's the speed has to be 10%+2 to get fined, i.e 3mph +2 = 35mph, 4+2 = 46mph @ 40 zone, got this info from police standing at roadside with a hand held device
"The guidelines recommend that action commences at speeds which are 10% + 2mph in excess of the speed limit (ie.35 mph in a 30mph area, 46mph in a 40mph area,etc). In this region, no action is taken by West Mercia Constabulary or the Safety Camera Partnership where speeds are below these thresholds. There are facilities in the guidelines which would cover action being taken at lower speeds where there are particularly aggravating factors, but the Safety Camera Partnership has never made use of this facility and has no plans to do so."