Any high street shop will be pricey, and if it's cheap it's probably end of line, but then you are paying for the sales staff, brightly lit shop and all that stuff. PCworld will take that to the extreme. Your local computer shop can be a better bet, but they may not have as wide a selection. Of course by buying at a shop in theory you are paying for the advice the staff can give you?
If you know what you want then online shopping will always be cheapest -
www.dabs.com,
www.simply.co.uk and so on have always been good for me, but check the delivery charges first.