Prices are in American dollars ($) as of today...
According to Google, £400 is equal to $586.92

CPU- AMD Phenom II X4 - http://www.newegg.com/Product/Produc...82E16819103471
$126

Mainboard- MSI 790XT-G45 - http://www.newegg.com/Product/Produc...82E16813130265 $90

RAM- G.SKILL 2GB (2x1GB) DDR2 1066 - http://www.newegg.com/Product/Produc...82E16820231144 $60

HDD- Samsung 750GB SATA - http://www.newegg.com/Product/Produc...82E16822152227 $55

Video- XFX ATI 4770 Radeon 512MB/128-bit - http://www.newegg.com/Product/Produc...82E16814150369 - $105

PSU- OCZ 500w 20+4 pin power connector - http://www.newegg.com/Product/Produc...82E16817341012 $55

Optical- Sony DVD burner SATA - http://www.newegg.com/Product/Produc...82E16827118039 $21

Case- - Rosewill Destroyer - http://www.newegg.com/Product/Produc...82E16811147144 $50

Total cost WITHOUT shipping fees: $562

I would recommend another RAM upgrade eventually, jumping it from 2GB up to 4GB. Also, the case is totally up to you. I picked the Rosewill because it that was neat, had decent reviews, and I like the top fan set up.

You could also run a RAID 1 setup for the hard drives and buy two and mirror them as a fail over but thats optional.

I would also look into an APS or Tripp Lite UPS as an accessory to help save your investment from a power surge.