"But Greece isn't even in the top ten in terms of external debt to GDP ratio. The UK has twice the level Greece has. It has everything to do with larger countries being able to borrow in the international market."
You disregard the private portion of external debt. The UK is a financial powerhouse, it and its companies owe money to other countries but other countries also owe to the UK, that's why its net international investment position is only slightly negative. As far as I see, all the countries with higher external debt/GDP than Greece has positive NIIP or only slightly negative, which means they (and their residents) own assets that might be easily liquidated to pay for the debt. The only exception is Portugal, which, just like Greece, has huge public debt, high external debt and pitiful NIIP. In fact, abysmal NIIP is what is common across PIGS. In Portugal, Greece and Ireland because of their public debt owed to foreign investors, in Spain because of foreign liabilities of its banks. That's also why the discussions of Spanish need of foreign help has little to do with actual public debt and everything to do with its ballooning in case they would have to bail out the banks. Spanish numbers are far from worst as a whole but it is the concentration of the bad numbers in a systemically crucial sector like banking that causes problems - it is also something you cannot discern by simply looking at a table.