It came about through legislation rather than case law. I think I just need to explain a bit further because that's not quite what it does. It says that if there's a substantial measure of conduct that took place in the U.K. then the forum bar applies. That's the gateway. That generally means that it could be prosecuted in the U.K. because a substantial measure of the conduct has taken place there. Then there are a number of specified factors that the law looks at, but there's no hierarchy to them. The court can give different weight to different factors. The factors include what prosecutions have taken place, whether there's an interest in prosecutions taking place in one country and the defendant's connections to the U.K. There are a variety of factors.
One of them looks at the possibility of prosecuting in the U.K. Now a U.K. prosecutor can say that there is no possibility of a prosecution in the U.K. and can issue a certificate. If they issue a certificate, you can't invoke the forum bar. That does give a prosecutor an ability to stop you from invoking. That hasn't happened yet in any case in the U.K. The prosecutor can also issue what is called a belief letter that says they don't believe the most appropriate jurisdiction for prosecution is the U.K. and they will then explain why they don't believe that. That happens in most cases and the court will then take that into account.
The reason why the forum bar is difficult to operate is the defendant always has long-standing connections with the United Kingdom and the court is trying to reconcile that with whether they can be prosecuted in the U.K. If they were already being prosecuted in the United Kingdom, they couldn't be extradited. You would ordinarily have a defendant saying that they want to be prosecuted in the U.K. and the requesting country saying that they already prosecuting the person and could do so swiftly. For example, they have other defendants who they are already prosecuting.
It's quite a difficult picture for a court to consider when weighing up these various factors. Often, the court is put in a position where it feels like it has to order extradition because if it doesn't do so it may be concerned that the person is going to have impunity and isn't going to be prosecuted anywhere.