For components and peripherals I've bought from Overclockers, Aria, ebuyer, Scan, Dabs, Komplett, Amazon and quietPC in the past. I've also looked at CCL - though never bought from them.
My current prefered retailer is ebuyer, and has been for at least 18 months. Even when ebuyer aren't the cheapest, it's normally only a few pence so the option of free postage when you spend over £50 usually rebalances total cost in ebuyer's favour.