This is a bit of a murky subject as it seems there are no fixed guidelines about this. Some say that if the car is worth less than £2000, then it should be exempt, then again some say that even if the car is only worth a few hundred pounds but is not essential for work or health reasons, then realistically the OR can take it. My personal experience is that my car is 13 years old and worth at most £500. The OR said they wouldn't be interested in it as it is old and not worth anything to them, but someone could buy it for me for a nominal amount of £100.
Sorry if I can't be more exact, but I think this kind of thing depends on your OR and what they decide, especially if the car is not worth very much.