if your parents live in a cabled area, ntl or telewest would be your best option, offering 10mb connections and as far as im aware, no download limit only a fair useage policy
So basically, are you saying your parents are ready and wiling to switch to another provider, based on your research? If not, I'm afraid you don't have any options (unless you get an extra phone line put in). You'll be stuck with whatever they've got.