For £700 you can get a Dell Studio 17 with 2.26Ghz core i5 (boost up to 2.56), ATI HD5650 graphics (latest mid range, will play most games), 2GB RAM (cheap to upgrade to 4GB yourself) and a 17.3" 1600x900 LED backlit display.
That would be for gaming, alternatively you can probably get a better CPU, more RAM and worse graphics for the same price if that's what you're looking for.