The police can stop a vehicle for any reason. If they ask you to stop, you should always pull over when it is safe to do so. You are breaking the law if you do not stop.
If you are stopped, the police can ask to see your:
If you do not have these documents with you, you have 7 days to take them to a police station. You are breaking the law if you do not show the requested documents within 7 days.
The police can also give you an on-the-spot fixed penalty notice for many minor offences and make you take a breath test in certain circumstances.
You can also have your vehicle seized if you are stopped on suspicion of driving without insurance and for some other offences.