Cameron's playing for time. What use is a promise of a referendum some time in the next seven years, but only if the Tories win the next election. He might as well promise to have a referendum some time in the next seven years to bring back the death penalty, but only if the Tories win the next election.
In both cases, in a referendum, the public are likely to give the response he doesn't want. Neither does Miliband think any differently.
It's an empty political gesture and a fudge to prevent the electorate from having a fair say on whether or not to stay in.
At that election, vote UKIP and force the issue.
How would voting UKIP force the issue? It would take a massive and wholly improbably vote shift for them to get a single MP in Parliament in 2015 under our electoral system, let alone enough MPs to hold any kind of influence after the election. That may not be fair (it isn't), but it is the reality of the situation. They may win enough votes in some areas to cause a few Tory MPs to lose their seats, but given the Tories are the only party offering a referendum anyway, that's a self defeating outcome if a referendum is what you're truly after.
David Cameron's biggest problem here is what would actually happen if he did
win the next election on the back of this referendum promise. The new influx of MPs that would give him his majority are likely to be on the right and anti-EU, adding to the sizeable number of such MPs already on his backbenches. Yet he has said he would personally campaign fiercely to stay in the EU when his referendum is held. Whatever the outcome of that referendum, he therefore cannot avoid splitting his party asunder and destroying his own premiership. If the result is to stay in, his rightwingers will blame him for their 'defeat' on the issue and seek to undermine him at every opportunity thus destabilising his government, as they did to John Major. If the vote is to leave, his leadership will be shorn of all credibility anyway and the battle for the succession will eclipse any kind of sensible policy making for the remainder of the Parliament.