Brightwells do a lot of the Dealer, Council and Police etc vehicles; Joe Public can buy there, but I'd recommend watching the sales in advance like a hawk - the results will tell you what the prices are doing compared to CAP Black Book trade in prices - and just remember you are bidding against dealers and there is no warranty. I've seen things go through for £x and go up for sale a week later for £2x on Autotrader etc (OK, at the £1000 end of the market) but it gives you an idea as to what they've paid for them sometimes... #
As far as I'm aware they're not insurance vehicles - if they have a history it's disclosed in the listing as Cat C or Cat D recorded - but some of the larger fleets self insure so you do get the occasional non-runner in the form of a Nissan Navara or Land Rover _________; on the whole though they have a bit of everything going through.