Final answer:
The British refused to pose for the portrait depicting the treaty negotiations because it symbolized their defeat and surrender to the United States.
Step-by-step explanation:
The British refused to pose for the portrait depicting the treaty negotiations because it symbolized their defeat and surrender to the United States. The portrait would serve as a reminder of their loss and the terms they had to accept. By refusing to pose, the British were avoiding any further humiliation or acknowledgement of their defeat.