CustomPC Magazine have a selection of parts for the various ranges (budget, mid, performance, and extreme). I'll just list their budget system, which runs to £524.07 inc. VAT.
Case: Silverstone PS02B Precision (£40.89)
Motherboard: Gigabyte GA-H55M-UD2H (£74.67)
CPU: Intel Core i3-530 (£89.99)
Heatsink Fan: Arctic Cooling Freezer 7 Pro Rev.2 (£17.50)
RAM: 2x2GB DDR3 OCZ Platinum OCZP1333LV4GK (£83.99)
Graphics: Sapphire HD 5770 1GB (£129.24)
Power Supply: Corsair CX400W (£39.25)
HDD: 320GB Seagate Barracuda 7200.11 (£35.80)
DVD: Samsung SH-D162D (£12.74)
Obviously some of the parts can always be replaced. 1TB Samsung Spinpoint F3 is currently £60.72, which is the best 1TB HDD you can get. If you get a more powerful gfx card like the HD5870, you might want to think more about a 600W PSU, but between them you'd add another £150ish to the total price. Also if you want more than a dual core Core-i3, then you might think about getting the quad core Core-i5-750, which costs £158.63. Then there's the case... the listed cast is roomy and has enough fans to keep it cool, but the awesome looking cases tend to cost over £100. My favourite case, the Antec Twelvehundred, is huge but it has amazing cooling power, at £117.37. On the other hand, the Antec Ninehundred-two is only £89.82. In any case, the more you spend on the parts, the better PC you get.
One thing worth noting - Core-i3 and Core-i5 are both significantly more powerful than most AMD CPUs for the same price range, but if you're looking to save as much money as possible, there are some fairly cheap AMD CPUs. Also, for most games you'll be fine with a HD5770 or even a HD5650, which cost around the £80 mark or below, but if you want to play Just Case 2 with the graphics reasonably high or some of the new games coming out these years, it's always good get the best graphics you can afford - it's future proofing so that you don't have to upgrade again. My HD4870 still serves me very well (equivolent to the HD5770 in number of stream processors, aka cores).