I've just been 'baited' by a dark green unmarked police Vectra on the M4 around Reading. They sat on my tail and flashed their headlights, hoping to push me into speeding away from them. The story gets a bit outlandish from there and is subject of a complaint.
I've seen a black and a red VXR on this stretch of road (and a blue standard Monaro), so I guess there are others in the 'club' who could face the same problem. I recommend that you watch out for tailgaters in this area who try to push you into speeding (not that you would of course).