A blink-and-you'll-miss-it Shout-Out occurs repeatedly with the two guys running the front desk at the police station. One reads nothing but Iain M. Banks and the other nothing but Iain Banks novels. This is a subversion of... something... in that while Iain M. and Iain are the same man, the two identical desk sergeants are not.
They just act like an evil cult, wearing black robes and chanting ominously, in a castle courtyard in the dead of night.
The name of Janet Barker's twins announced at the NWA meeting, Roger and Martin; after the protagonists of the Lethal Weapon series.
The use of The Cardigans' song "Lovefool" in the village staging of Romeo and Juliet implies that they've taken their cues from Baz Luhrmann's Romeo + Juliet (1996).
More obviously perhaps, Blower and Draper are using the famous knight/angel costumes from that film. (While the background characters are in period dress), and Juliet uses Romeo's Beretta to commit suicide.
The priest seems to have spring loaded pistols in his sleeves a la Travis Bickle.