I quiried this with a traffic bobby, he said they dont like it but there is nothing they can do. Apparently the only reg regarding use of lights is you cant show a white one on the back. As long as they come on with the marker lights its ok
Not sure, to be honest. Certainly the traffic cops in Liverpool will pull you -- one of my mates got pulled and they ended up phoning up his insurance company in a (failed) attempt to get them to withdraw cover. I had to write (he is Czech and can't write English at all) a grovelling letter to his insurers on his behalf.........
One of the reasons they hate it so much is that their own plain clothes cars are externally distinguishable only by the flashing blue lights under the grill (and maybe by a second aerial).