In England the justification has to be approved by a Custody Sgt initially that gives up to 24 hours. A more senior officer has to review the continued detentio

In England the justification has to be approved by a Custody Sgt initially that gives up to 24 hours. A more senior officer has to review the continued detention from the 6 hour mark up to 24. A Superintendent or above can authorise up to 36 hours. Beyond that a Magistrate can authorise up to 96 hours in 36 hour blocks. During this time the individual has access to legal advice, where the reasons for continued detention have to be provided and the evidence thus far collected explained (again unless a very senior signs off on why not). Unless you're been arrested for a very serious crime, in which case you're going to be defended for free by a very experienced QC who will tear apart dodgy process, you are likely to be arrested for a handful of hours. The exceptions would be being held overnight, if you're drunk/high and thus incapable of being interviewed etc. Things are slightly different in Scotland where you get detained not arrested, but the principles are broadly the same. UK policing as a whole has been through periods of dodgy processes and huge reforms in the wake of them. There are likely to be very few criminal justice systems in the world I'd rather interact with than the UK one, especially if I were accused of a serious crime. Edit to add - plus the premise of "fishing" implies there is no existing evidence. The initial justifications for the arrest have to be supplied at the start, and "it might have been him" won't pass muster.