If you don't mind a refurbished laptop (one which comes with a whopping three year next business day warranty), check out this Dell Inspiron 14Z ultrabook, available from technology_outlet, an eBay reseller, for £345 with free delivery and 354 Nectar points.
The Dell Inspiron 14z runs on an Intel Core i3-3217U dual core processor clocked at 1.8GHz with 3MB cache and an Intel HD 4000 Graphics (which means that it is a third generation Core model). Other features include 4GB of RAM, a 500GB 7,200 RPM hard disk drive (plus a 32GB system SSD), HDMI, Bluetooth 4.0, Wi-Fi, a DVD writer, a pair of Skullcandy stereo speakers, a 14in 1,366 x 768 pixel display, a card reader, a HD webcam, a 6-cell battery life, Windows 8 and a weight of 1.87Kg.
Granted that the Dell Inspiron 14z is not what one could qualify as a pure Ultrabook (one which doesn't come with a spinning hard drive or an optical drive) but it is a fairly affordable model and one which carries a rather substantial warranty. Check out our review of the Deal Inspiron 14Z Ultrabook here.