The UK has a problem: it has universal jurisdiction for war crimes and crimes against humanity. This has led to several embarrassing incidents where people have taken the law at face value, and demanded that the government prosecute war criminals visiting the UK. In the absence of any government action to do so, some have even tried to enforce it themselves, by bringing private prosecutions against visiting war criminals.
This has caused a great deal of embarrassment to the UK government. Why, they can't play host to some very good friends with blood on their hands, while joint meetings on how better to effect war crimes have to be cancelled for fear that someone might end up in jail. So they have a solution: require the Attorney-General to consent to any prosecution. That way, she can veto any attempt to hold war criminals to account, and the law can go back to being what it was always intended to be: a dead letter which is never enforced (or at least, never against friends and allies - a tool for victor's "justice" or colonial "justice", nothing more).
This is how power protects its own. And its a clear example of why we should regard the powerful as hostis humani generis, the common enemies of all mankind.
