You’re nearly right. The job of the police is to investigate and then submit a report, where there is evidence, to the CPS (COPFS in Scotland).
Should the police believe there is sufficient evidence for a criminal case, it’ll then be sent to the director of the CPS (the organ grinder, not one of the monkeys) to decide if it proceeds to a law court.
So, if it ever gets to the end point, any investigation will require;
The cooperation of all involved. That is all witnesses and suspects.
The willingness of the Met to actually do a proper job, bearing in mind the Dame in charge is possibly up to her neck in it herself and is, at the very least, politically compromised due to the nature of the job of MPS commissioner.
The Director of the Crown Prosecution Service (CPS) to not shirk his/her responsibility in taking criminal proceedings against civil servants and/or government ministers, effectively their boss(es), and not be swayed by the promise of a nice gong in the upcoming queen’s jubilee honours.
That’s all before it gets in front of M’lud and M’lady that comprises the justiciary.
Is that a bit too cynical?