But what if Greece doesn't pay?

From today's Open Europe news summary:
Mail: Greek exit from the eurozone could cost the UK economy £105bn
The Mail reports that, according to unnamed senior government sources citing internal Treasury calculations, a Greek exit from the eurozone could see UK GDP fall by £105bn (around 7%). The article notes that this estimate is much higher than estimates by financial firms, with much of the cost stemming from a fall in UK exports to the eurozone and collapses of European banks causing losses for UK banks.

An export is simply a sale to a foreign buyer, for which the exporter must be paid. If Greece cannot pay its debts, it makes no sense to continue export sales, just as it makes no sense to sell to a domestic customer who does not pay. Greece cannot pay for its imports, except by borrowing or printing euros. Of course the export industries in the UK want to make sales and, undoubtedly, lobby the government to lend their fellow citizens' money to Greece in order to fund these sales, but this means that all UK citizens are paying their own export sector to produce goods for Greeks. The same is true of Germany, where powerful labor unions in the export industries lobby the German government to continue funding the many EU bailout funds so that they can keep their jobs. It is a giant scam, perpetuated by fallacious Treasury calculations.