The Post Office and its numerous subsidiaries are ultimately owned by The Crown and like many other government agencies they do not have to refer to other government reporting agencies - the police, CPS etc when prosecuting.It's a great TV drama, but how the PO were able to take staff to courts for prosecution without reference to the police or CPS is beyond belief, that's got to be changed.
I actually find it strange that most folk appear to lay the blame at the door of Fujitsu when they (Fujitsu) were merely contracted to provide Horizon by the customer who was The Post Office - whilst I have no doubt there were defects in what was delivered ultimately it is the customer i.e. The Post Office who agreed to accept this system as fit-for-purpose and who then allowed it to be rolled out.