Was this on the A1? It looks like Peel's van that they use at Doncaster and Liverpool airports to capture anyone stopping (yes stopping, not parking) where they are not supposed to.
As they can only work stationary, because they measure relative speed and have no cross reference to vehicle speed, it’s either a flight of fantasy, total bks or something else was flashing.
It's the same as those used for parking / stopping enforcement around airports . I do find these things annoying in the way they are all dressed up to look like police vehicles at first glance , but only just err on the side of legality .