Hopefully Dell caught up with switchable graphics otherwise forget using it for travelling long distance. Same with the i7 version, even with the 90WH battery. Also 15'' are kinda heavy and bulky for travelling. the 56WH might be a bit short as well for watching movie (I'd wager, it's about just over 2 hours worth of juice for movies with a i5, even less with no switchable graphics).
I'd also consider splashing out for the 1080p version and bluray if that's your thing, but it'd be adding £250 on top!
Basically, I'd be weary using an XPS for travelling, I'm not sure they are designed for that type of thing. If it's got a switchable graphics and you go for dual core (i3-i5), which have integrated GPU's for saving batteries, and if you want to use the XPS for travelling while not being afraid of weight, then I'd get the 90WH battery.
The GT420M and a i5 will play the latest games at medium to medium-high settings, but not you'll have to cut down on some fancy stuff. Also gaming at a native 1080p resolution will eat a bit of framerate compared to a 720p resolution.
Also gaming on batteries probably not advisable. You'll get 2 hours gaming at most even with the extended battery, once the dedicated GPU and CPU throttle up. It will also put strain on the battery.
EDIT : This is purely speculative given the specs. If you can find a review, then that would be better 