...well led by BC, the CPS guidance on this is:
http://www.cps.gov.uk/legal/p_to_r/public_justice_offences_incorporating_the_charging_standard/#a37
and the relevant bit is: Impersonating a Police Officer
Section 90 Police Act 1996 creates several offences relating to the impersonation of police officers or the possession of articles of police uniform, namely:
impersonating a police officer (including a special constable);
making a statement or doing any act calculated falsely to suggest membership of a police force;
wearing a police uniform calculated to deceive;
possessing an article of police uniform.
The circumstances of the case may disclose more than one of these offences. It will seldom be necessary to charge more than one offence. You should select the most appropriate.
You should consider the motive of the defendant. Where the impersonation involves a threat to the safety of any person, or to property, or is done with a view to financial gain, then a prosecution should follow.
I would have thought he had defences to all of these....