LONDON — British Prime Minister Rishi Sunak was struggling to avert a leadership crisis on Thursday after his plan to revive a blocked asylum deal with Rwanda triggered turmoil in his party and the resignation of his immigration minister. Sunak called a hastily scheduled news conference to defend his policy after Immigration Minister Robert Jenrick quit the government late Wednesday, saying a bill designed to override a court block on the Rwanda plan “does not go far enough” and won’t work. [time-brightcove not-tgx=”true”] Jenrick said the government had pledged to “stop the boats” bringing migrants to Britain across the English Channel and must do “whatever it takes to deliver this commitment.” The plan to send asylum-seekers on a one-way trip to Rwanda is central to the U.K.