I think coppers have to show you a warrant card if you ask to see it. Also as far as I know off duty coppers can still do you.
Did you see that program on ITV last night (you've been framed thing with a different name) some guy had to remove a scarecrow dressed as a copper with a speed gun sat on top of his hedge as it was impersonating a police officer and they weren't impressed. Report him anyway and get back to us.