Fake police warning notes?

We know it's illegal to pretend to be police, but is it illegal to put out fake police warning signs?

Look at this sign here in Cliftonwood, telling off a car for parking on the pavement.

Now look at the car. One wheel up. Over in Monty the fact this car was sticking so far out would merit attention -that it was stopping through traffic, not supposedly interfering with pedestrians. It has to be fake.

We aren't naming names, but we do note that John Grimshaw of Sustrans lives nearby...