Asda do some really nice pretzles. Low saturated fat (the stuff which sticks around longest) low sugar, nice flavour. Not sure on their actual name but they're yummy and good for you. Come in salted (tastiest me thinks) or cheese and chive (stinky.)
Rice cakes? I can't stand them myself but I know they're healthy as they're basically pure air.
Although pretty **** to eat seeing as they have a really high glycemic index..they'll boost your blood sugar levels but only for a short time and then they'll plummet and you'll be really hungry.
Nuts, especially honey roasted cashews and peanuts, yummy and filling. Actually, though, in saying that cashews are quite expensive, so maybe just peanuts?