Useful things to look for in the UK, in order as vehicle closes

The most obvious sign - very bright headlights (although most Impreza turbo drivers go for the same type:-)

Antennas non-standard for type of vehicle

Anything suspicious in the front grille - okay, sometimes it'll be a strange mod by a teenager, but best to play it safe

Police cars almost always have 2 occupants if on duty

Police cars tend to have 2 rear-view mirrors

Police cars very rarely have any stickers in the back windscreen - and definitely no dealer stickers.
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On the topic of speed limits, I am more than happy to drive 20mph in town, but on a wide motorway with no traffic in the middle of the night - 70 is nonsense. There was a guy stopped a couple of weeks back at 140mph in a 70 zone, was let off because the judge and jury agreed he wasn't driving dangerously, and I have seen 180mph done safely in certain situations.

The Association Of British Drivers states the case best. And until the govt make sensible speed limits, I will almost certainly use my own judgement on safe speed when out of town. If that means keeping very alert, using detectors and playing the game with Mr. Plod, then so be it