I have never been sure how accurate GPS speed is. The raw satellite signal is a time signal. What the gps is doing is finding your location from the time signal of several satellites, leaving a short time lapse and then locating you again. Your speed is distance between the tow locations divided by the time gap. So there are three obvious sources of inaccuracy - the locations ( typically to 2m radius on my marine GPS at best) and the time ( which is pretty good). You then need to add in atmospheric effects , signal delays etc so you end up with a combination of errors. Over a longer time interval the speed will be more accurate than a short time interval.

Wouldnt worry anyway. The thresholds on most speed cameras are pretty generous. An old friend ex plod who was the traffic man covering the Avon motorway bridge for some time set the threashold there at 20 mph above the limit because and less than that used to flood them with cases. I suspect its the same over here because drivers have been ignoring the permanent 50 on the smart motorway for years and the message would have got out by now if it was anything like 55 or 60.