Ambassadors to the United Kingdom are not called that officially, but rather Ambassadors to the Court of St. James.
The Court of St. James, or St James Palace, was the palace which was the residence of the monarch before Buckingham Palace was built. It still remains the official residence, although Buckingham Palace is the official London residence.
The Court of St James was chosen as the venue for accreditation of ambassadors and High Commissioners in that the Marshall of the Diplomatic Corps, who acts as the link between the Queen and the foreign diplomatic missions, is permanently based at St James Palace.
Dr Sir Lesley Colin Patterson,
Cultural Attaché to the Court of St James and Chairperson of the Australian Chapter of the International Cheese Board