Is there enough evidence against the defendant – reliable and credible evidence, enough to provide a “realistic prospect of conviction” is it in the public interest for the CPS to bring the case to court – bring a prosecution unless the public interest factors tending against the prosecution outweigh those tending in favour of a prosecution.
Private prosecutions can be brought in relation to any alleged criminal offences, but the most common types are related to:
Costs – you're able to claim back cost from the court even if you lose or withdraw the proceeding.
Defendant costs - unlike other proceedings, generally you will not have to pay the defendant cost if you lose your case
Speed - unlike civil proceedings, matters can be brought to court very quickly
Control - you are in control (not a government agency)
Expert legal resources - you have access to a better resourced, more focused and efficient team rather than any other public prosecution
Wider powers of punishment - criminal convictions carry a stigma whilst criminal courts have wider powers to punish from fines and confiscation to imprisonment.