I would be surprised if you were able to stick to a maximum budget of £300, WF, but as Sandra said you'd have to be looking at onboard GFX and sound to cut costs. Onboard sound isn't a problem, but make sure you get a mobo with an AGP slot, so you can upgrade the GFX at a later stage. I can practically guarantee you will want to, as most budget boards don't carry 'gamers' GFX.
Don't forget, it's the bits you don't think of which bump up the costs, such as Case, Fans (CPU and Case), Leads, Modem, Keyboard, Mouse, Speakers, and what about a Monitor? You're talking about £70 for a budget CRT, which is going to knock quite a hole in your £300. Sorry if this isn't what you wanted to hear, but being realistic, to start from absolute zero, you've got to be looking for a minimum budget of about £500.
Have a look at some online retailers, and build yourself a shopping cart in each, to compare costs. If you're going to buy online, it pays to get most of the stuff from one or two retailers, so as to avoid extra shipping costs. Some of the cheapest I have found are Watford Electronics, Redstore, Novatech, CCL, and Dabs.