Can the police stop you at any time?
In Britain the police have the absolute authority to stop any drivers they wish for any reason. This could be as mundane as a traffic census as well as more serious matters such as crime prevention or investigation.
They do need to have a reason to stop you, and you are entitled to ask what this is. Random checks are not permissable.
If you are asked to stop by a uniformed police officer, you have to do so, as soon as a safe opportunity arises, otherwise you commit an offence. If you genuinely believe that to stop immediately would create a potentially dangerous situation you should do so as soon as possible and report to the officer who flagged you down.
An officer is entitled to ask you to produce your driving licence, MOT certificate and certificate of insurance. Failure to do so immediately is not an offence but you could be given seven days to produce them at a police station of your choice. Whilst failing to do this would constitute an offence the production of MOTs and insurance certificates is demanded less often these days since the required information is usually available online. However, if the officer/s believe that you are not insured (they would check the Motor Insurancer Database, which should carry details of your policy) they have the power to impound your car on the spot and have it towed to a police pound.
If you were stopped because they believed that you had committed an offence, they may issue a fixed penalty notice. This usually happens if the offence is relatively minor, including, for instance, careless driving, 'tailgating', driving whilst using a mobile phone, failing to wear a seatbelt, or failing to ensure that passengers wore one. The penalty could be a fine of up to £200 plus penalty points. This is not inevitable however, and some forces prefer, depending upon the offence, to issue warnings, or offer a driving course to first-time offenders. If the offence is considered to be serious enough, such as driving in an aggressive or dangerous manner, they can impound your car.
If you are issued with a fixed penalty notice you may decline to accept it. In this case you would face a court hearing, at which higher penalties may be applied.
If you were suspected of drink driving you would normally be required to take a breath test. Failing this test would result in your being ploaced under arrest and taken to a police station, where a further test would be carried out.
If the police find minor defects to your car, such as a failed headlight bulb, you can be issued with a 'vehicle defect rectification notice'. This gives you 14 days to have the defect repaired, and produce proof at a police station that this has been carried out.