Ultrabook vs laptop: Is an ultrabook worth £500 more?

The ultrabooks offer 2 main benefits: better design and higher portability. So if you care about the design a lot or you have to carry it around with you all the time, then you might want an ultrabook. Future ultrabooks will also have much better IPS and higher resolution displays. So that's a big one, too.

I wouldn't really consider the "extra performance" from the SSD a benefit, because you also get significantly weaker CPU's and GPU's, so at best it's a wash. You also get less storage, unless you get a hybrid with at least 64 GB SSD for Windows and cache, and a HDD for regular storage.