As Ethel's link shows, the police can stop your vehicle at any time but they can only search it if they have good reason to believe that it contains stolen goods, drugs or weapons. (They can also search it for 'prohibited items'. For example, if someone was released from prison, on licence, with a condition that they shouldn't possess alcohol, the police could search their car if they received a report that the person had just left an off-licence with several bottles in a bag).
However, that link doesn't mention the provisions of the Terrorism Act. Under the Act, the Home Secretary can designate certain areas, for limited (but renewable) periods where the police can stop and search anyone (including drivers and their vehicles) without any specific suspicions of illegal behaviour. The areas currently covered include all airports and railway stations as well as roads within central London. So, if you're driving along the Embankment, the police can stop your vehicle and carry out a search purely at random.
Chris