How about Japanese rice crackers? I am currently munching my way through a bag of them - they are VERY crunchy and almost fat free. This particular brand is called Sunrise and the ingredients are:
rice (85%) soy sauce sugar tapioca starch
I think I got them in Waitrose, but I've bought other (similar) brands in Sainsbury's and HFS.
Also, I'm sure I've seen a thing for slicing potatoes very thin to make homemade crisps - maybe in the Lakeland catalogue? - and I don't see why you couldn't then bake them in the oven with a spray of oil. Has anyone tried this already?
Have to say I don't agree that we in the UK are badly off for IBS-friendly foods - there may be fewer junk food substitutes, but from what I've read on these boards, it seems much harder to find bread and other basic foods without HFCS or artificial sweeteners in the USA than it is here. The one thing I really miss is cheese and unfortunately there's no substitute for that anywhere ...