Anyway. Go to the local schools they will have some spare. I know my local one offer them for around £20 for a 2.4ghz P4, 256MB of RAM, integrated graphics. I think it is a 40GB hdd.
I may get one for a HTPC and change the case and use RAM from my PC in it when I upgrade but I really want one of their old servers.
Go to www.gumtree.com, then click on the links on the right hand side of the page that corresponds to the city you live in. It's pretty much like the Loot. I've got a PC advertised on there for about 80 quid, might be willing to negotiate. I'm based in London though, I guess it depends on where you live in the UK.Hope it helps.