Citing talks that have made little progress, workers on the London Underground and Britain’s national rail network have planned the first national 24-hour strike in more than a decade for June 29. The strike was originally scheduled for June 10 but called off because of promising talks with the government, but now the strike has been approved by union members. It will begin at 6:29 p.m. on June 29 and end at the same time the next day. The strike, if held as planned, will cause significant inconvenience throughout Britain, but especially in London.