Drivers may be under an illusion here; lorry drivers in particular seem to routinely do 56 in motorway roadworks 50 limits (measured by my sat nav, not speedo) - and, on you tube there's at least one dash cam video clearly showing this (ironically, showing alleged 'bad driving' by other drivers!) , and, when questioned, the driver stated it was 'within the limit'.
Isn't the 'concession' to reflect that no measuring device is 100% accurate? I don't know what the prescribed accuracy of the camera is, but it can never be + or - zero. So, if it's 10% out (overstating) then you pass it at a real 50, records it as 55, you're ok because it's within the tolerance. But if you pass it at 56, it records you as 61.6 and you are caught.
Not just the accuracy of the speed camera, either; the device the driver uses to indicate the vehicles speed will not be an exact. But the lorry driver mentioned above obviously thought that his dash cam was +/- zero. for speed indication.
PS I think there is an obvious defence for exceeding a 20 mph limit (slightly - say 23). I reserve the details in the event of my own defence but here's a clue; look at the C&U regs