Mens rea (or intent) is important for most crimes. So, in order to impersonate a police officer, you've got to intend to do so, mostly.

(There are a few crimes where it's not true, and you can be done without having the intent to commit a crime. Carrying a knife in a public place is one of them. You are allowed to carry with a reason, but if you don't have a reason, then carrying the knife is an offence, even if you didn't intend to be offensive ;) )