LONDON — Prime Minister Theresa May said Monday that the postponed vote in Parliament on Britain’s Brexit agreement with the European Union will be held the week of Jan. 14 — more than a month after it was originally scheduled and just 10 weeks before Britain leaves the EU. But even as May insisted she could salvage her unpopular divorce deal, pressure was mounting for dramatic action — a new referendum or a vote among lawmakers — to find a way out of Britain’s Brexit impasse and prevent the economic damage of a messy exit from the EU on March 29 with no agreement in place. Jeremy Corbyn, leader of the main opposition Labour Party, said he would submit a motion of no-confidence in the prime minister over her delays.