This screening is part of a Crime Hong Kong style season at the Barbica. You can find all the details here.
Here is the Barbican introduction:
The Dowager Empress’ ruthless security chief is sent to the south to recover stolen palace treasures and hunt down its thieves. As his men are killed one by one, he finds himself questioning his faith in the system he has vowed to uphold. A period set crime film; The Killer Constable bridges the gap between the Shaw Brothers studio films of the early 1970s and the more cynical approach of the Hong Kong New Wave. It is considered by many as director Kuei’s finest hour (and 38 minutes).
Here (and above) is the trailer.