Perhaps the law about warning others predates the SatNav which makes it a bit of an anomaly. To be honest, the police have always said they are there as a deterrent rather than fine collecting and if you flash your headlights at them, you are acting as a deterrent too so you could be said to be aiding the police. 
I do try to watch my speed, especially in built up areas, but the one time I did get caught was when I was being distracted by my passengers and I hadn't realised as I pulled away from the roundabout to catch up the cars ahead that I was going fast enough to warrant a Speed Awareness Course.
Prince William on Graham Norton!
Wes Streeting announces we have 1,000 days to prepare for war.
