In general, under the provisions of the Police & Criminal Evidence Act 1984, your car can only be searched where there is a valid reason. For further details, go here:
http://www.leeds.ac.uk/law/hamlyn/searchp.htm
Then scroll about half-way down the page, until you reach "Police and Criminal Evidence Act 1984, s.1
Power of constable to stop, search persons, vehicles etc."
However, under the provisions of Section 44 of the Terrorism Act 2000, there are circumstances where both individuals and vehicles can be stopped and searched with the need for any element of suspicion. The 'circumstances' referred to must be defined by the Home Secretary but, for example, a person's presence at a London railway station is currently regarded as sufficient reason for the police to stop him and conduct a random search. Similarly, driving a car through central London is also currently a valid reason for random 'stop & search' procedures to be used under the Terrorism Act.
Chris