Actually having something with "Northumbria Police" written on it isnt the impersonating part, because he could have just had a letter from the police before and just cropped out the rest of the letter, but kept the part he wanted, and stuck it in his wallet. He shows it very quickly to the cyclist and unfortunately the image (even the raw file the cyclist will have) will probably be out of focus in some way. But showing this to the cyclist isnt an offence, calling yourself a "serving police officer" is the offence.
But I too am very surprised the police didnt take this any further. Oh wait, no I am not because they dont bother to take anything further nowadays because it is too much hassle for them.