The police in England and Wales make a decision to investigate after a complaint is made or evidence comes to light of a possible offence.I think at this point it’s worth thinking about the relationship between the press and the police, and why only specific people are arrested and investigated for possible corruption while others are not. I mean, who is making the decisions here and why.
The CPS would make a decision in a case in such a case as to whether charges are appropriate. The DPP has given guidance as to when the police need to seek advice.
In Scotland I believe the police would seek advice from the Procurator Fiscals office about charges.