On the surface Shock! Horror!
But the police do not prosecute anyone ~ that task falls to the CPS.
If the police believe that they will not be able to persuade the CPS to prosecute a case then they will not tell the offender that. What they will do is "Offer a Caution". The offender can, of course, refuse to accept a caution but few will do as they believe that they will escape court and subsequent publicity by accepting the caution. A police caution is not a let off as it carries with it a entry on a criminal record (often a first for the offender) and his/her card is then marked for all time. The police, for all we may mock them at times, are not stupid. Rather a caution and a criminal record than have the offender walking away because the CPS says there is "No chance of securing a conviction".
Sensible use of Police Powers is often the best course of action in these cases.