The Ghost: How can I get back to the mainland?
Barry: Only by plane, I'm afraid.
The Ghost: I lent my jet to my butler.
Barry: Haha, oh you Brits!
When the Ghost writer stops the BMW to follow the sat nav there is a car in the background. It vanishes then reappears as the film changes from close up to panoramic shots.
McGregor finds that the BMW navigation system is active, and decides to follow the directions, having no idea where it will lead. There are obviously much easier and direct ways to check the entered target destination of a sat nav than actually following it. Instead of this being a mistake, it is most likely a script choice; portraying McGregor as stressed, technically unknowing, inexperienced driver of modern luxury cars, or all of that.