I only use websites as stores will rob you for every penny you have got, I have seen graphics card for sale at 2 times the online price. The websites I mainly use are scan.co.uk and amazon.co.uk. Amazon is surprisingly competitive with the pricing and variety of components also most postage is free. I have also used ebuyer and eclipse computers and nothing bad to report from them.
Scan and overclockUK are terrible. Aria is pretty good when it comes to returns but I mostly buy from amazon due to returns and faulty issues get resolved a lot quicker.
Scan is the best retailer ive used. They have a store in Bolton near the football stadium in Horwich. They sell prebuilt if you dont want to build your own, and also components if you know exactly what you want.