i built mine for under �200.
i'm in the watford area so if that's local and you have the abilty ( or a clever pal ) you could also. depaending what spec you require and what extras are needed it could be less or loads more. mine's a 3.2 processor & motherboard with new bits and old bits added on.
You could try looking on your local Freecycle group website. Look on Yahoo, in Groups. I see people offer computers all the time on my local group, tho might not be the latest version (!), or people might only offer them in bits, (i.e. just the monitor, or a PC without the monitor etc) but you could get it all separately.