Britain’s political strains burst into the open when Defence Secretary John Healey resigned after a fierce row over military funding, charging that Prime Minister Keir Starmer had failed to back the country’s security needs at a moment of growing danger. The surprise departure, delivered alongside a sharply critical public letter, adds to the pressure bearing down on Mr Starmer as he confronts the prospect of a leadership challenge. It also lays bare the government’s central dilemma: how to increase defence spending when the…