I think the best way is either you build it yourself or go to the local store and ask them how much will it cost if they help you to put all hardware together. (I assume you get all Hardware yourself) I knew my local store charge £20 for checking, building new one, may be £50-100??
Anyway just give you some idea what you can get...(price based on ebuyer.com)
CPU: intel E6750 £115
MB: Gigabyte GA-P35C-DS3R £75 (if it is not availiable, get any P35 chipset with gigabyte, MSI)
RAM: 2x1GB DDR2 800 £50
Optical Drive: SATA DVD burner £20
HDD: Seagate/ WD SATA 320GB 16MB Cache, £50
GPU: 8600GTS 256MB £120
PSU: Any quality brand, e.g. Actec, Consair, OCZ, Thermaltake, etc. Get 500W min. approx. £50
Case: what ever you like. say £30
MS Home preimum 64bit OEM £62
19" Widescreen Monitor £120
Other: keybroad, mouse, speaker, case fan, thermal paste, etc. say £40
Total: £730.
If you don't need new monitor, just get a 8800GT instead of 8600GTS.