Well, i was in the same boat as you. I knew nothing about 3 days ago and have just been lurking these forums. It's worth building it yourself mate, you can mix and match the parts to suit your needs
As far as companies go, the system I am looking at purchasing is made up from Amazon.co.uk, Ebay.co.uk, Overclockers.co.uk, Ebuyer.co.uk and Scan.co.uk
Have fun searching.
Edit : The build I have put together.
HDD - Samsung Spinpoint F3 1TB - £39.50
RAM - GSkill RipJaw 8Gb - £44.99
Case - CoolerMaster Centurion 5 II - £89.99
PSU - CoolerMaster 650W GX PSU - inc. in above
GPU - MSI HD 6870 - £147.99
CPU - Intel i7 2600k - £210.00
Mobo - Gigabyte GA-Z68AP-D3 - £82.72
TOTAL - £615.19