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Flashing lights as warning of speed trap - Zippy123
The crime would be perverting the course of justice.

I cannot find the "official" answer at the moment but there was a case in 2005 where someone was prosecuted for doing this. The person was found not guilty as it was proved that he could not know if the cars he was flashing were speeding or not. If he did then he would have been guilty.

Flashing lights as warning of speed trap - Zippy123
Found a link:

www.honestjohn.co.uk/forum/post/index.htm?t=35892