Be carefull here Jim. You have stated, The only related law is "UNLAWFULLY PURPORTING TO BE A POLICE OFFICER"
That is incorrect and you should read up on the Police Act 1996 sect 90 (3) which clearly states the offence of possession of any articles of Police uniform for persons who are not Police or Special Constables.
Also it can be argued,and people have been arrested for Theft when caught in possession of Police articles, particulary current issue stuff which technically belongs to the relevant police authority and should not be sold on the open market.

Thank you to Mike for pointing out Section 90 of the Police Act 1996. If you take the wording of section 90 it would appear that any person who is not a Police Officer or Special Constable, would be guilty of an offence to possess any article of Police Uniform or Insignia. However it would be correct to point out that the section specifically relates to IMPERSONATION.

If it were to mean anything else outside of the realms of IMPERSONATION, then a lot of Police Officers would be in trouble. Including Chief Constables who have authorised the sale of their surplus uniform to such companies as "Onestopcopshop", or who have authorised the giving of Patches and Badges to collectors who ask for them. Who have authorised the sale of Helmet Plates to rasie money for charities, the list goes on.

Would the literal interpretation of this act mean that Police Community Support Officers, Civilian Staff, Cadets, Security Guards and any other person who lawfully wears a uniform similar to a police officers also be guilty??