£500 is a tall order for a complete system with the kind of spec you'd want. If you're capable, a self-built machine using parts might be an economical route, as you can pick and choose where your money is going on each component. Overclockers is a food website for this, and you can ask on the forums for suggestions on exactly what to buy for any given task and budget.
I've never had good service from Scan. I'd advise avoiding them. Ebuyer are good.