I saw one just outside Wolverhampton, near Wednesbury on the A41 BC route today, he'd pulled a car over.
At first I thought it was an accident "car vs. bike", until I noticed the flashing blue/red light at the back integrated into the tail light. The bike itself was dark grey, looked a bit like a VFR from the fairing (but what do I know)
There were road works so I had a nice slow ride past him to have a good shufty. There were no police markings on the bike of any kind. The only indication that he was a fed was a checkered band on his lid, which he'd left on the seat. I couldn't see anything on the front so that flasher probably built into the fairing or headlight or something.
Chances are you would never know this was police til it was too late. You probably race past him and even give him a wave.
Safe to say my sensible hat was well and truly on from that point. I just wished I'd put my helmet cam on this morning.