This has been bugging me and, as I work in this field, I've just had to look it up!
Under the Police Act 1996 s.90(1):
Any person who with intent to deceive impersonates a member of a police force or special constable, or makes any statement or does any act calculated falsely to suggest that he is such a member or constable, shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months or to a fine not exceeding level 5 on the standard scale, or to both.
S.2 makes it illegal to wear a uniform to pretend to be an officer too.
But not retired officers! So knock yourself out, sixlostmonkeys, though I can't for the life of me imagine what pretending to be a retired officer would achieve 