Would you want something portable than you can take to Lan Parties/Meets etc.. ?
For you budget I really would put it together yourself, you can get an awesome PC for your money that way.
YoYotech (who are in London incidently) are doing fairly decent prebuilds at the moment. Check them out.
For me though I would build the following.
Intel E5200 + Xigmatek tower cooler ~£55
DFI P45 uATX ~£105
Silverstone TJ-08 ~£55
Corsair 520W HX ~£65
640GB Western Digital AAKS ~£48
Samsung Optical drive ~£15
4GB PC6400 DDR2 RAM ~£50
Vista home premium (64bit if you can and OEM) ~£65
Nvida GTX260 ~£170
2 x Scythe slipstreams ~£10
~£660.
That will be a silent (once fans are replaced with the slipstreams), small and cool running PC
Leaves £340 for the monitor and other gubbins.
£130 will get you a very nice 22" screen. £200 will get you a pretty nice but cheapy 24"
Personally I'd stick to the 22" though given the budget.
Which leaves just over £200 for Speakers etc.. Which is a hell of a lot really.