Are you prepare to build it yourself?
Here is the list from ebuyer.com. approx £475
E8200
http://www.ebuyer.com/product/139967
Gigabyte GAP35-DS3
http://www.ebuyer.com/product/136363
OCZ 2GB Kit (2x1GB) DDR2 800MHz/PC2-6400 CL 4-4-4-15 PLATINUM XTC
http://www.ebuyer.com/product/116755
Point Of View 8800GT 512MB GDDR3 256bit Dual DVI PCI-E Graphics Card
http://www.ebuyer.com/product/135258
Seagate ST3250410AS 250GB
http://www.ebuyer.com/product/131301
LG GH20NS10 20X SATA DVD1RW/DL/RAM - Black Bare Drive OEM
http://www.ebuyer.com/product/141833
OCZ StealthXtream 500W PSU
http://www.ebuyer.com/product/135159
Microsoft Windows Vista Home Premium OEM - DVD - 64-bit - English
http://www.ebuyer.com/product/123067
Case not included. Get what ever you like. But make sure it have some case fans.