I don’t know whether you’re asking (a) if you’re allowed to go more than 50mph (so long as you average no more than 50) or (b) if the cameras will only detect you if you average more than 50 mph.
The answer to the first question is emphatically “No”. The limit is 50mph and you must not exceed that even for a short distance, or you risk prosecution.
The answer to (b) is “Yes”. The camera times your vehicle over the distance they are apart and calculates your average speed over that distance. So you could travel at five minutes at 100mph, pull over to the hard shoulder for five minutes (making sure you have a good excuse to be there) and you would only have averaged 50mph over that stretch.
Average speed cameras were introduced because people were simply slowing down to pass a conventional camera (which only measures your speed over a very short distance) and then speeding up afterwards. A good example of how they are sometimes abused can be seen on the M6 Toll road in the Midlands. I believe the entire stretch is policed by average speed cameras and it is not unusual to see drivers pull over into an area just before the Toll Plaza at the northern end to “wait for time” having tanked it at excessive speed. Quite why anybody should do this is completely beyond my understanding.