I don't think they do flash My hubby has just been caught by one, he got the 3 second of 'mobile' warning on the road angel and slammed on the brakes, it was too late by this time, he was caught doing 84 in a 70. The letter took about 3 days to come through. This was in Cambs on the motorway, the cop van was on a brigde about a mile away when it caught him, fookers!

BTW, it's only motorways and dual carriage ways where we would go a bit faster.