I work for an executive corporate transport provider (ok, cab firm) in Northwest London and I would say at least half of our company cars
and driver-owned vehicles came from
The Great Trade Centre in Hythe Road, London NW10. I understand they have other locations too and you can check out their inventory online at
http://www.greattradecentre.co.uk/
They're as cheap as chips and for the most part the vehicles they sell are trouble-free. My own car came from there in 2001 and (touch wood) I have never had a problem with it that wasn't the direct result of my wife bumping it into a something or someone. I should perhaps mention therefore that I've never had to trouble their after-sales service personally but I understand their 'customer care team' can be something of a challenge. For strict value for money they are hard to beat.
One word of advice: don't bother with their extended warranty - it's not worth the paper it's written on.