If the police stop you, keep a cool head
Being pulled over by a police car can be unnerving experience. It's important to remember that for the police officers, this a routine procedure, and that they do not want to make the process unnecessarily difficult for you.
What are you legally obliged to do?
Police (or traffic) officers can stop you without giving a reason, ask for your name and address, and ask you to show them your licence and the vehicle's insurance and MOT certificates. To ask you for more, they must have reasonable cause.
Establish why you have been stopped
If the officers have not made clear why you were pulled over, it is perfectly appropriate to ask for the reason, politely of course. If they suspect you of a traffic offence, they must say so.
Fixed penalty notice
For minor traffic offences such as speeding, using a mobile phone, or driving too close to the vehicle in front, the police are empowered to issue a fixed penalty notice; to reduce pressure on the courts, fixed penalty notices are nowadays used to enforce various kinds of 'moving traffic offences' which can also include traffic light offences and restricted turn contraventions. You do not have to accept the FPN; it is a conditional offer, meaning that you can accept guilt and pay the fine, in which case you will get the appropriate points on your licence. Your alternative is to refuse the offer, after which you will receive a summons to appear in court. Fixed penalty notices are only issued on the spot if the driver's licence is available; otherwise, a conditional notice is issued, and the licence has to be produced at a police station within seven days.
If you are arrested
It is rare for the police to arrest a motorist who has been stopped for a traffic offence. However, if they suspect you of a serious offence, they may arrest you, in which case you are likey to be taken to a police station. The reasons for the arrest, including the alleged offence, have to be made clear. Once you are under arrest, the police can search both you and the car. You will be informed of your rights, including legal advice and the opportunity to inform someone.
Above all, it's important to remember that you can only make your position worse by being threatening or aggressive, by blustering, or lying. Being related to the chief constable has rarely been found to be helpful, even if it's true! Remember also that minor incidents are sometimes dealt with by means of advice or a warning - police officers can do this at their own discretion, which they are much more likely to exercise in your favour if you appear to understand that you have made a mistake, and are willing to accept what they say to you.