If you owe 5000 pounds you have to worry. If you owe 500 billion pounds the bank has to worry. It's like the situation with Murdoch in the late 80s when Sky TV was losing billions. The banks had no choice but to keep lending.
Europe's leaders, particularly Germany, are trying to hold a gun to Greece's head. They threaten that if Greece tears up the austerity package they will kick it out the Euro.
Greek voters were never consulted on austerity and 70% of them have now voted for parties that reject it.
As I have previously explained it would be economic madness and cost far more to Germany et al to kick Greece out. Who thinks the markets would stop at Greece? No, me neither. For this reason I believe the Germans are bluffing, just trying to scare Greek voters to back the bankers medicine.
Also Greece will not leave the Euro voluntarily. The 17 nations would have to break European law and break their treaty with Greece to kick it out.
Of course the IMF, ECB and EU could refuse to loan Greece any more money. But as Alex Tsipras of the left party expected to gain victory in Greece's June elections says; Greece would simply default on its current loans. It would be impossible for them to do anything else. Greece has more power than most think. The markets can throw a hissy fit but ultimately governments have the power to overule.
As to Cameron, he is trying to play the statesman - it briefly worked the Gordon Brown. He is an annoying heckler from the sidelines to EU leaders. But he is also a powerless irrelevance. Even the Tory media will struggle for a positive spin.