There's quite a few places you can spec up your system and purchase off the net here in the UK: Scan, Aria, Overclockers and CCL spring to mind, but local computer shops often offer a build service, too.
As has already been said, you can get better value by assembling a system yourself, although it may seem daunting at first it's actually quite simple these days, apart from a good quality Phillips screwdriver all you really need is a little common and the patience to read the manuals that come with the parts before beginning.
Try looking up a few 'how to' vids on YouTube to see what I mean.
And to USAFRet: I wager 500 Quatloos on the newcomer building his own. 😉