David Cameron knows better than I do, that the UK will not default on any of its debts, because the Pound has been a fiat currency since 1948, and so the Bank of England can and will simply create whatever number of pounds are deemed necessary to be able to meet any and all obligations. (That is why we have a rampant welfare state and massive public sector in the UK, in spite of the fact that we cannot afford them in truth, it's because we don't have real money and the government can print up whatever it needs to offer you whatever you want in order to win your vote, and thereby retain what they really want -- which is power.)
In fact, for those of you who are not watching at home, I can report to you that they already are doing exactly this, in broad daylight and on a massive scale, but they call it 'Quantitative Easing' so you hopefully won't twig what they're really up to — they're already 'Doing A Zimbabwe', 'monetising their debts' by paying them all off with freshly minted money and thereby steadily debasing the value of all money previously in circulation. In other words, they are stealing from you and me, and the other people in the world who have made the mistake of buying our debts to keep as their own currency reserves for their own countries (you know who you are, China). The government of the UK will only be able to get away with this charade for so long, before the value of our currency drops like a stone on the world stage and we descend into third world poverty. But that can't happen here, right? Yeah, yeah, please save it -- I've heard it before. :-\
No, David Cameron isn't warning everyone that Labour might cause the UK to default on its debts. He is using the economic disaster that Gordon has propagated over the last decade through his misguided fiscal policies, to score political points. He is trying to better his own future position, by feeding the nation blatant lies that few among the population care to think about and understand. It ain't rocket science, but we've all been taught to believe even thinking about it is crass and distateful.
Sadly the only mainstream UK politician who seems to understand the economics of the situation, and what to do about them, perhaps even is personally ready and willing to do it in spite of the fact that it will be political suicide even if it is the only good thing for the country, is Vince Cable. Its just a shame he is a member of the Lib Dems, because they are unelectable in every other respect.