i'm not bothered, but i have noticed that the police seem to be able to make the rules up as they go. i know that in the usa they need a warrant to search your property, weather it be your house or your car. But speaking from personal experience, when i was 17 and driving home i was stopped for no reason and told to get out and they searched my car and found nothing, i doubt they would do this if i had been say 50.
Although not really pertinent to a "routine" check, the Police can also search a vehicle under Section 60, Criminal Justice and Public Order Act 1994, whereby a police officer of the rank of superintendent or above may authorize all persons and vehicles within a locality to be searched regardless of suspicion, if serious violence is expected in an area. This power may be exercised by an inspector if he believes violence is imminent and no superintendent is available.