Just to add my tuppence:
I've been to Big Motoring World 3 times in the last 5 years or so, once to look at a car for myself and twice with two different friends.
In one case the car wasn't available, it had been sold and moved off site in the 3 hours since we'd phoned about it.
The second time (for me) the car made my head spin. Alarm bells ringing everywhere. Mismatched tyres, slightly suspicious service history documentation (looked like they used the same pen for every service over 5 years on an E46!) and some of the panels were different colours and had clearly been painted in the last few days, and yet they claimed the car had no accident history and was in original paint.
The third car was again a bit of a joke. Tyres, paint, interior trim, etc.
Then the service history showed services up to 93k miles ending a couple of years previously yet the car only had 70k miles on the clock, and they were advertising it as a 70k miler.
Big Waste of Time.