Not familiar with the cost of these components in the UK (besides conversion of currency, not sure how much these are in terms of value). But I'm assuming you built this to make a little profit. Check the Dells and the HPs and see what they go for similarly spec'd and then try to undercut by a little. Over here, I'm guess-timating you could get $1350 for it.
Just a word of advice, don't sell it on ebay.uk. You'll have problems shipping the thing (without damage) and once you do, you risk buyer scam. What I mean is the person will receive you rig, say that you shipped them a box of bricks and force paypal to give them back your money. So you'll be out your rig and money.