An SSD certainly if it's going to be carried around a lot, and if I had around £700 to spend I would buy a Sony Vaio model. They are available in Ultrabook spec. and you should find one within your budget, at least from an online supplier anyway.
I've always found Sony build-quality & reliability to be top-notch. Not exactly cheap but you get what you pay for in technology goods.
I'd second the Sony Vaio. Vaio T's are their ultrabook line, however their Vaio S series are almost the same size/weight with more processing power (maybe outside your budget though). I know the Vaio S has a removable HDD, not sure about the T; but in general installed SSDs are rediculously expensive, you're better off adding your own from newegg.