I know of no laptop under £700 that will have a soundblaster soundcard. However have you considered a PCMCIA expansion card to use in any laptop or a USB one?
This means you can concentrate on getting the best laptop you need and then when it comes to selling it just keep the card and ditch the laptop for a new one. Laptops are not really designed with sound in mind.
However do you honestly need a soundblaster card? I use on board sound of my laptop and find it more than good enough. A friend of mine who is a DJ uses his gear through his laptops inbuild parts.