C'mon, BRL, don't get paranoid. I'm just putting my point of view, doesn't mean it's right and you're more than welcome to tell me I'm talking b@lls - it's you that's buying, not me
Space is one good reason for wanting that sort of machine, although you would probably still do better (IMHO) with a small-footprint PC + TFT monitor. Still, I just find that people tend to buy laptops without thinking about what it is that they want, and often spending more money than necessary because they know that bigger numbers are better.
My take on it would be to go to somewhere like Dabs and search for every laptop under, say, £700 including VAT (there's a very useful-looking Fujitsu, for example, at £675 all-in). Find one you like the look of, then ask yourself what it doesn't do that you need. And if it isn't what you want, go buy that fuggin' thing you just listed - me not knowing why you'd buy it isn't a reason...!
edit: I don't know what sort of seating you get on a train (off-duty, I assume!
) but when I did Chichester <-> Brighton in cattle-class every day the 12" screen, 1.2kg Vaio was the perfect laptop. There wasn't room to use anything bigger.