Directing the police - who's the boss?

There is a long-standing and well-established principle that the Crown and the police are independent from one another. 

At the investigative stage, the police are independent of the Crown (both in the sense of the Crown Attorney's office and, more generally, the executive branch as a whole).  Neither can order the police to lay a charge or not lay a charge.

However, once a charge is laid the prosecution is controlled by the Crown, independent of the police.  The police cannot direct the Crown to continue a prosecution or prosecute in a particular way.