It's the only case when you can legally cross a junction to make way for an emergency vehicle. You can be prosecuted if it’s a fire engine or an ambulance. But if it's a police car your defence is that you were instructed to cross the red light by a police officer. Obviously any magistrate with common sense would look at the facts and if the driver had no other way to get out of the path of an emergency vehicle he would dismiss the case against the driver. Of course, if getting out of the way caused a crash then a driver who went through a red light would be held responsible, just as the driver of an emergency vehicle would be.