One of my friends uses one of these ->
Clicky
And he swears by it. If you are looking to save money one of these would be a good option as they come with pretty much everything you need onboard i.e. high quality onboard audio, onboard graphics (GF4 MX400 performance), onboard lan.. the list goes on.
Only problem is i guess that only leaves you with about 60 for processor, hard drive, RAM and cd rom.
btw, will you be needing a monitor? because that will be around £100 for a decent one.
ALso here are a coupel of sites you will probably want to look on as they are the best for PC stuff in the UK imo.
http://www.ebuyer.com/customer/home/
http://www.aria.co.uk/
http://www.komplett.co.uk/k/k.asp