Green’s Dictionary of Slang

a policeman or police squad specializing in the entrapment of gay men.

1990		F. Wheen Soul of Indiscretion (2001) 190: The young officers who act as bait are sometimes known as ‘pretty police’.
1998		P. Tatchell speech at Keele University 24 June 🌐 Huge resources were invested to arrest gay men committing victimless homosexual offences. [...] Agents provocateur ‘pretty police’ were deployed to lure men into committing homosexual acts, and then promptly arrested them!
2002		Socialist Outlook No. 56 🌐 Police raids on cruising areas are frequent. This may involve entrapment, where plain clothes ‘pretty’ police pose as gay men waiting for sex (this is particularly the case around public toilets).