Putting a PC together is a lot easier than most people first think, just ask anyone in here who has done it.
I would never buy a ready made PC from anywhere unless it was from a friend who had just brought a new one.
You would be much better off buying all the parts you need and putting it together yourself.
are a good place to buy from, so are www.dabs.com
If you know exactly what parts you want, then its all about getting them parts for the cheapest price. try ebay for some last minute bargains on hardware.
When you have all the parts you need check out www.buildyourown.org.uk
for advise on how to put it all together.
Ask some of your friends if they know anything about how to put a PC together, you might be surprised.