Surely anyone flashing their lights and "observed" would use the simple defence that they were flashing for one of its two legally sanctioned purposes - to indicate "I am here and perchance you appear not to have noticed", or "I intend to overtake".
The "observer" would then have to testify that there was no possibility that the intended recipient of the signal had failed to notice either the presence of the flashing vehicle (representing a potential danger or obstruction) or was signalling an intention to overtake a slower vehicle. And as we all know, coppers never lie under oath.
And as we all know, coppers never lie under oath.
Many years ago when I had a passing interest in UFOs, there were some stories of police seeing these objects. When two police officers are in agreement and testify in a court of law, the odds are you’ll get a conviction every time. However, when they testify about seeing flying saucers, those odds drop to zero.