For between £500 and £600, you will get a gold plated laptop these days !
I would mosy on down to PC World and have good browse. If you really want to pay all that money, get the best spec you can find, although you could spend a lot less and still have a good machine. I have used Tesco Direct a lot recently, and you can collect it from your nearest store, which beats staying in waiting for the postman !
http://www.tesco.com/direct/technology-gaming/all-laptops/cat3376345.cat?icid=computers_textnavstamp_slot1
Some of these will be available in-store, in a big Tesco Extra.