Green’s Dictionary of Slang

(UK prison) Queen’s Evidence, i.e. giving evidence against one’s fellow criminals.

1989		J. Morton Lowspeak 66: Going QE – giving evidence for the prosecution against former colleagues.
2003		Observer Crime 27 Apr. 28: Q.E. Queen’s evidence. An accomplice in a crime giving evidence in the hope of a lighter sentence.
2011		J.J. Connolly Viva La Madness 90: Some of these fellas nowadays...They go QE like that.