Judicial review available for unreasonable/irrational decisions
'A confession of a crime is no good without a complainant.'
Absolute rubbish. Why don't you sit in a court for a week or so and find out? Plenty of people get convicted on their own confession in interview when the victim refuses to give evidence.
'Incidentally, how could McKinnon's lawyers "launch a judicial review of the DPP". They could request one, but without any evidence that the DPP has acted unlawfully they have no chance of getting one.'
The above poster clearly does not completely understand the rules of evidence, let me further enlighten their expertise on the subject of judicial review of administrative action.
If a government department (here the CPS) take a decision that is irrational or unreasonable then that decision can be judicially reviewed. You do not, as the poster asserts, have to show that the decision was purely unlawful.