No worries. Well, I'd recommend something with a Core i7 or a quad-core athlon.
A portuguese forum has several guys that advise people on various budgets.
This one
NOX ZEN 23,40
CORSAIR CX-430W* 43,10 €
ASUS M4A77T/USB3* 73,50 €
KINGSTON KIT 4GB DDR3 1333MHZ* 27,80
SAMSUNG 500GB SATA II 16MB F3 36,60
SAMSUNG DVD±R 22X
SH-S223 PRETO BULK (SATA) 17,30
AMD ATHLON II X4 640 (3.0GHZ) BOX SKT AM3 87,00
POWERCOLOR HD6770 1GB DDR5 PCI-E* 106,00 €
(Prices obviously in euros) will be about 520 pounds.
Some stores will even assembly it for you for about 20-30 pounds, but for this I can't provide an answer as I don't know the reliable stores. A Dell or something based on these specs will be a lot more expensive, probably nearing double the price.
Plus you'll have warranty on individual components, and can change the parts without invalidating it (unlike my Acer desktop).
Another example, for about 600 pounds.
COOLERMASTER ELITE 370 - 40.90€
CORSAIR CX-600W V2 - 64.50€
GIGABYTE
GA-Z68A-D3-B3 - 126.80€
INTEL CORE i5 2500 (3.3GHZ) SKT 1155 - 176.20€
GSKILL KIT 4GB DDR3 1600MHZ RIPJAW X (CL9) - 33.80€
POWERCOLOR HD6950 1GB DDR5 PCI-E - 207.60€
SAMSUNG 500GB SATA II 16MB F3 - 36.60€
LG DVD±R 22X H22N PRETO BULK (SATA) - 19.80€
(sorry for not editing the prices, I'm on a rush)
and
Thermaltake V4 Black Edition - 41.90€
XFX Core Edition PSU PRO 550W - 58.90€
Asrock Z68 Pro3 - 110.90€
Intel Core i5 2500 3.3GHz - 167.50€
Corsair 2x2048MB DDR3 1600Mhz XMS3 CAS9 1.65V - 37.90€
Gigabyte GTX560 Ti OC 1024MB DDR5 PCI-E - 226.90€
Samsung 500GB Sata II 16MB F3 - 35.90€
LG DVD+/-
Rw H22N Bulk SATA Preto - 18.90€
about the same price. Depends on what you find, for what prices, etc. Hope it was helpful. If you want to check out the forum, it's
http://forum.zwame.pt/forumdisplay.php?f=152 . It's in PT but Google auto-translate is your friend