A friend of mine got fined for doing 71mph in a 70mph zone under similar circumstances. He opted for an expensive one day road safety course instead of a direct fine and licence points.
On our UK motorways we are seeing the latest HADECS3 cameras. These cameras are super accurate. They are the ones that are on a gantry shelf on the side of the motorway and not the traditional overhead type. They are also variable so if the speed limit changes due to traffic ahead, so does the camera.
At the time, the normal car speedo calibration was about a 6% to 10% overread, by law, so when he was caught the letter said his speedo must have been reading over 75mph so no defence.
However, this 6% to 10% overread is now reduced in new EU cars from 2021, again, by law, as they now have safety systems that can read roadside speed signs and automatically react. My new car has such a system, if I choose to activate it, which I don't. I also noticed the far more accurate speedo.
My 2007 Honda Civic speedo was well out, at GPS 70 it was reading 77
My 2009 Civic was slightly better and read 75
My 2013 Civic read 74
My new Honda Jazz, at GPS 70 reads 71mph.
My GPS figure agrees on my Garmin DS61, my Viofo dashcam and phone speedcam app.