I'll say this as politely as I can

If you've no idea what you're doing, then a car auction is not the place for you.

For starters, you don't mention if it's saloon. Estate. Coupe. Convertible. Manual or autobox. Spec, again, will make a HUGE difference to price. As will condition. (BCA rate cars on condition, 1-5). Service history.

Without all of those pieces of information, then nobody can even begin to guess.

Then, you need a table of charges for a private buyer at BCA. Prepare to be horrified. Here is a photo showing what BCA fees were a year ago (so they could be different now)

postimg.org/image/439kmg485/

(Thanks to DaveyK_UK for posting a link to that information originally)

When you add in the auction price, the buyer's fee, the DVLA documentation fee, (possibly) VAT, you'd almost certainly be better off buying a car from a forecourt.

Finally : Remember, absolutely no warranty at all on the car, other than it started and went into the auction hall. If the gearbox dies tomorrow, it's YOUR problem.