OcUK have in my experience provided good customer support. Even when things went wrong.
Ordered parts for a new PC last year which all arrived fine, and no issues since I built it.
I once ordered a gaming headset which they had on their website as in stock, but when I ordered it the stock went to 0 and then was physically taken off the site...
I e-mailed them to say that if it was so much of a problem I would just get another one, and they refunded my next day delivery as it took a day or two extra to arrive.
In regards to pre-built systems Overclockers have been doing it for many years now, obviously you can't omit the possibility of receiving a machine that falls into the small percentage of systems that arrive with problems, that's just luck and not all problems can be identified soon after the system is built. I would purchase with confidence.