Certainly our representatives should take into serious account the views of their constituents. It seems to me that this is what happens, most of the time. But cases such as NJ's Jelly Baby tax don't usually come up -- in the sense of such universal support for a given measure, I mean -- so, mostly, the views of constituents are too mixed for any one MP to be able to reflect them all in a single vote. If an MP gets two letters from two constituents, one imploring the MP to support such-and-such a position, and the other equally passionately against it, then how can that MP vote to reflect both? Abstain, I suppose -- but then very little would ever get done because almost always those two constituents would exist.
Or, at least, that would be true for the legislation anyone outside Parliament actually cares about. The other catch is that there's a lot of stuff in Parliament that goes unnoticed to the (majority of the) general public. But that is the second part of representative democracy: we elect these people to do this stuff on our behalf so that we don't *have* to care about such things, most of the time. And, again most of the time, that's fine and dandy. It needn't be an issue for public consideration to decide whether to set a tax on Jelly Babies or not, and then at what level under what conditions. Should the tax scale with weight or be fixed at 11% to the bag? Or 12% a bag? Or should it be flat for the first 99.9 grammes but after that start at a base rate of 5% and then jumping a further 5% at 500g and again every 500g after that, reaching a maximum of 45% for a 5kg purchase? (Wow, even thinking about 5kg of jelly babies makes me slightly nauseous... )
But the silly details above aside, no way can such a matter be put to the public. The blunt instrument of a referendum can't cope with that. But then, in a representative democracy, it doesn't need to.