As far as I understand, the law with regards to emergency vehicles is this: They have lights and sirens to warn motorists of their impending presence. However, the sirens are not a "Get out of my way" thing. Just like the horn. They are simply to make you aware of their presence.
As a result, you have no legal obligation to move over or stop. You should only do so when it is safe and convenient. It is up to the emergency vehicle to pass you safely. They CANNOT bully you out of the way.
Take your time when these vehicles come behind you, you dont want to panic and be another casualty because it could put the person the emergency vehicle is responding to in danger.
If the vehicle does not have any sirens or lights on, then the vehicle is classed as any other car on the roads. They are subject to red light offenses, illegal overtaking, speeding, and undertaking. Therefore, if you see one breaking the law, you have every right to report it to the police. Unless it is a genuine emergency, the vehicle should not be putting other people in danger or breaking the law.