As with most of the other replies, I'd recommend doing a custom build (better value for money etc...)
If you're new to the whole concept of custom building, I'd recommend using a tool like Lambda-Tek's Custom PC Designer to check hardware compatability (and then you can shop for parts elsewhere).
I've attached the link below.
In terms of stores you can buy parts from, since you're UK based, I'd recommend OverclockersUK, Scan and Lambda-Tek, and for quick price comparison try using `The Invisible Hand`.
http://www.lambda-tek.com/computing/pcdesigner.htm