Yes BCA is the place to go, got my van from there, ex southern electrical. Full service history, 100,000 miles, 1 previous owner (bit high but I paid very little for it) all sorts of vans there.
You go, bid on the vehicle you want, pay £500 straight away. then go and pay the rest at the counter.
best to go first and see how things work, make sure you go into the area where they start the vehicle, check that the vehicle starts first time, i.e. have a look before it goes into auction room.
The same type of vehicle can go for a massive difference in price depending on time of day.
i.e. vauxhall vivaro, ex bt van, 90,000 miles can go for £6,000 at 10 am and another one exactly the same can go for £9,000 at 11 o clock.
reason is that there is dealers there and they will go for the best inside condition one, as well as mileage, i.e.
I would never go to a van dealer as I know they get the vans from there and you are paying Atleast £1,000 on top of the price you could pay on there.
just remember,
1. VAT on top of sale price ( on most vehicles)
2. The auction fee on top of price, based on the final sale fee.
like i say best to go first to see how things work, they are every week, so you can go back the following week to buy.