What exactly is it that the police should have done? Place her under house arrest? Monitor her 24 hours a day? (do you have literally any idea how difficult it is to obtain a warrant to search the house of a known criminal once, yet alone monitor a 15 year old girl for 24 hours).
This page is from APP, the official source of professional practice for policing.
www.college.police.uk
Take her into care? Remove her passport? What?? Do you want to introduce new legislation which makes it easier for police to detain minors on suspicion? Because you're very long of accusations of corruption and culpability, very short on actual solutions.