By Mark Kleinman Sky News’ City Editor Mark Kleinman writes a weekly column for City A.M., this week on Nationwide, the ‘British ISA’ and Martin Gilbert’s latest venture NATIONWIDE MUST ACT IN MEMBERS’ INTERESTSOne member, one vote? Not in the case of Nationwide, Britain’s biggest building society, whose £3bn takeover of Virgin Money, Sir Richard Branson’s financial services brainchild, stunned the City last week. Even more astonishingly, Nationwide’s 16m members won’t be allowed to have their say on the biggest corporate move in the mutual’s 140-year history. Its interpretation of the Buildin…