Have a look on www.specialreserve.net and get their mag, if your a member (cost is £7) you get everything in their ex.VAT. If you got your self a monitor and keyboard/mouse, you can build your self a half decent PC for under £200.
PC World /) have got some good offers on where you can get your self an entire PC for about £300. Also have a look at www.europc.co.uk & take a look at Dell's site. I was thinking that the Dimension 2400 would be ok for you. £241 +VAT. There's also www.ebuyer.co.uk and www.dabs.com