I played on one of the best gaming laptops a while ago. The MSI GX60, it has a 7970m videocard which is similar to the 7850 (desktop) graphics card. On the system I was playing the laptop was connected to three 1920x1080 screens running some new game which I did not know. Graphics were decent so it was definately impressive. You could buy it for €1200 I believe so in the UK it will probably be around 1100 pounds. This does actually beat some desktops when gaming.
For a desktop.
GTX 560 TI + i5 3570k would be great for what you plan on doing. Completing the full build it will cost you around 650 pounds I'd say.
Another laptop around the same price range as the desktop.
690 pounds i5 3210m + 630m videocard + 8gb ram + 1TB HDD. Will play WOW nicely at around 60 FPS on medium settings I'd say. Good for multi tasking aswell