In Bedfordshire the police wanted to start fining people for going 71 with average speed cameras and they were told to jog on by the government. So there is definitely some leeway. I think they usually allow up to 79 before you get a fine.
The cameras are not 100% accurate, so if they cut it too fine it probably won't stick.
I'd have thought an average speed calculation over a large distance would be VERY accurate. unless their clock/timing method is completely broken. All you need is synchronised time and two reference points.