Who's going to be setting the questions? Presumably the government? Which means realistically the Conservative party or Labour (generously assuming they can recover to a position of electability at some point). Why would they then ever pose referendum questions where they wouldn't like one of the answers (especially in light of the last referendum)? How does anyone really get more of a democratic voice when you'd just be choosing from options selected by the 2 big parties so that their preferred option is all but guaranteed to win?
"Should we: A) waste £90 million on a complicated new voting system, or B) keep our traditional voting system and spend the money on the NHS?"