Living in London I have to say that the car auctions here are all pretty crap. I've just returned from a BCA auction in Enfield (N.London) were most of the cars were either pretty knackered or else over-eager bidders were paying close to book price (perhaps there are more newbies on a weekend).
What part of the country would be best for car auctions? I wouldn't mind taking a train to Leeds, Bristol etc.. if I have a better chance of getting a good (sub-£5,000) motor.