Collar Prevent Strangulation

I had a film on VHS long ago and can't remember the name of it. This film is set in victorian times. Women of a good social class are being murdered by strangulation. The film centers around the relationship between a police constable and a particular young woman (who I thought was kiera knightley?). The main characters sister and maid gets murdered and in the end she sets herself up for bait by wearing a specific collar around her neck (to prevent being strangled) and the killer is found to be the vicar's wife.