If Barack Obama were dreaming up the ideal start to Mitt Romney's first overseas visit as the presumptive Republican nominee, the president might wonder whether his rival could offend the US's historic transatlantic ally. That would obviously be rejected as impossibly ambitious, so the president might then ask himself whether Romney would fail to remember the name of one of his hosts in London. Surely a successful businessman would never make such a basic error. So the president would wonder whether Romney would breach convention by saying in public that he met the head of MI6, Britain's overseas intelligence agency....