I'm sorry that I can't answer all of your questions directly, but I used to work as a 'trade plater' (mainly delivering vehicles to and from auction houses), so I might be able to give you a few 'pointers'.
First of all, I'd never consider buying a car from BCA. They always seemed to me to be a totally 'shambolic' organisation. They seemed to get very few good quality ex-fleet cars to sell. Most of what I took to their sites was barely driveable.
In my opinion, nearly all of the good quality ex-fleet stock is sold through the giant (American-owned) Manheim network:
http://www.manheim.co.uk
Unlike BCA they've actually got a website which is helpful to new buyers:
http://www.manheim.co.uk/find_buy/guide.html#6
Even better still is to buy directly from the fleet owners. The biggest company vehicle fleet in the UK, with over 75,000 vehicles is BT. Obviously many of those vehicles are vans and lorries, but they've also got thousands of company cars on the road. They're all maintained to incredibly high standards (even higher than, say, most police cars) but, unlike many other ex-fleet vehicles, they don't get sent to auction. You can buy directly from BT Fleet:
http://www.btfleet.com/sales/cars-commercial-v ehicles.aspx
(If you get to know the manager at your local BT Fleet service centre, he'll probably keep an eye out for any particular vehicles he knows that you're interested in. Those managers receive frequent lists of what's available throughout the country, with many vehicles selling within hours of first appearing in the listings).
Chris