There are still a few issues about this that seem odd.
Firstly, I’m still very surprised that a ticket was issued for 55mph in a 50mph. The average tolerance level is 10% plus 2mph, which is 57mph. I don’t know of any police forces that don’t have a tolerance above 10%, even if it’s only 1mph. See link below where AutoExpress surveyed all Police forces in this. Some refused to answer, but none said that they have no tolerance above 10%.
Secondly, when a motorway speed limit suddenly changes there is a time lag before the camera will flash at the lower limit, in order to cover scenarios such as this where motorists haven’t had time to react to the change.
I wonder if the letter has a photo of the vehicle showing the speed that it was travelling at?
https://www.confused.com/on-the-road/driving-law/speed-camera-tolerances