The following advice is for anyone who feels vulnerable were he/she to stop. In many cases there may be several of you in the vehicle or you may be confident you can deal with the situation, in which case this advice may not apply to you.
An unmarked police car can stop vehicles but in order to comply with the provisions of the law, it must contain a constable who is in uniform. Failing to stop for a constable in uniform is an offence. In cases where a driver failed to stop and drove to the nearest police station/place of safety etc. before stopping because they were unsure of whether they were being asked to stop by a genuine police officer, if the police took action, it would ultimately be a matter for a court to decide whether they had committed an offence.
If a car flashing for you to pull over or stop is unmarked, unless you are certain it is the police, do not stop. Drive steadily to the nearest public place e.g. a petrol station where they are open till late, a police station or somewhere there are a lot of people, and then stop. If you are in a relatively deserted area, as a last resort, consider looking for a house that is obviously occupied and pull into the driveway. You can always apologise to the householder afterwards.
Try and signal that you have acknowledged the request to stop and indicate the action you are taking (put your flashers on or signal by pointing from the driver's window etc.). Don't drive off at great speed making the police think you are trying to get away.
Keep the doors locked until you are happy it is the police and have your mobile phone to hand just in case. You can ask to see the police officer's warrant card, which should carry their name and photograph, through the closed window.
Incidentally, if you are suspected of drink/drug driving none of these actions would invalidate an officer giving you a preliminary test, as you have only temporarily interrupted your journey and are still driving for the purposes of that law.