I think they fear that he has well-connected and organized supporters, something most other people fleeing don't have. And he has the benefit of time: he doesn't have to move until everything is perfect, because there is no way of catching up to him where is right now.
If you don't know when your target is going to move, how do you stop him from leaving UK waters before you even realize he isn't there anymore? Monitor every single boat moving on the Thames, 24/7? Search every ship leaving your jurisdiction?
Getting out probably isn't easy, but I would be surprised if there weren't quite a few people/organizations capable of doing it.