Well, firstly I recommend using these websites for components:

www.overclockers.co.uk

www.ebuyer.com

www.novatech.co.uk

Get a decent Motherboard so your PC will be easy to upgrade in the future,
you shouldn't need a quad core right now so a dual core will be alright, at least 2GB of Ram, I recommend from OCZ and an Nvidia 8 series Graphics card. I'm not the best person to ask, so ask Chalex as well. Hope you found this helpful.