In my day whether to charge a person with an offence was the decision of the Detective in charge of the investigation. The continuance thereafter and further or amended charges was the decision of the police solicitors dept, the barrister i/c the case and maybe the DPP.
As you correctly say the decision whether to find a person guilty or not guilty is that of the jury, as directed by the trial judge.
The law as it affects reasonable force is not a matter of statue law as decided by parliament but case law as decided by president in earlier cases.