Hmm.... If you can get the parts and bring them to Canterbury I can put it together for you over an afternoon (on a Wed/Thurs or weekend perferably). All I ask for is a 6 pack (or a 4 pack or whatever).
Otherwise I'll be in Guildford in April (EDIT: But according to Google maps I need to go to Heathrow first then backtrack? And it seems like it takes just as long to get to Guildford than it does to Canterbury?)... In that case all I ask is for train/coach fare to Brighton then return to Canterbury and maybe a meal (are fish and chips good there?). PM me if you want to work something out. For sure I'll be cheaper than any retail store and do just as good if not a better job. I take a lot of care when assembling, do cable management, and keep everything clean so that airflow is maximized, a concern especially with mATX computers since the parts are in a smaller package.
As for buying a prebuit.... You're going to get raped on the price of the parts anyways, the build below you'd probably pay a good $1100-1300 (CAD) for it prebuilt (like 700 quid for 400 quid worth of parts. The stores make a killing on these prebuilt gaming computers and the manufacturers even more by including all that bloatware) from a company like HP/Sony/Acer/Dell etc.
Try this, it comes to 402 quid and is pretty decent; Phenom 955 at 3.2 Ghz with an unlocked multi (with C3 stepping I've heard people get 3.8 Ghz overclock on stock voltages but YMMV), HD5770 which is no slouch IRT modern games (plays BFBC2 nicely according to my brother), 4GB RAM which is pretty standard for gaming computers, 1TB Hard drive for all your storage needs, and toss that all in an mATX package (18x35x44cm):
AMD Phenom II X4 955: http://www.overclockers.co.uk/showproduct.php?prodid=CP-244-AM
XFX HD5770: http://www.overclockers.co.uk/showproduct.php?prodid=GX-149-XF
Gigabyte 760G mobo (mATX): http://www.overclockers.co.uk/showproduct.php?prodid=MB-245-GI
4GB OCZ Intel Extreme RAM (Don't worry there's no such thing as "Intel RAM", it's just a marketing ploy and just means that it's validated for use with the Intel P55 chipset): http://www.overclockers.co.uk/showproduct.php?prodid=MY-188-OC
1TB Samsung Spinpoint F3: http://www.overclockers.co.uk/showproduct.php?prodid=HD-082-SA
OCZ 500W StealthXStream: http://www.overclockers.co.uk/showproduct.php?prodid=CA-045-OC
Coolermaster Elite 342 (mATX): http://www.overclockers.co.uk/showproduct.php?prodid=CA-229-CM
Samsung SATA DVD+/-RW: http://www.overclockers.co.uk/showproduct.php?prodid=CD-099-SA
Only things I would really change are the mobo and the case but since you have size constraints I'll just leave the mATX stuff alone. The other thing is that the mobo's chipset is outdated but IMO it's not like you're going to notice the difference between a 760G and an 890FX chipset anyways.
PS: I can also install Windows 7 for you, I have an extra copy of Win7 Professional edition that I need to get rid of.