If the president needed to settle a diplomatic situation with great britain, which cabinet position would most likely be sent to carry out the mission?

The United States Ambassador to the United Nations is the leader of the U.S. delegation, the U.S. Mission to the United Nations. The position is more formally known as the "Permanent Representative of the United States of America to the United Nations, with the rank and status of Ambassador Extraordinary and Plenipotentiary, and Representative of the United States of America in the Security Council of the United Nations"; it is also known as the U.S. Permanent Representative, or "Perm Rep", to the United Nations.