Hyde Park Corner is a busy intersection and traffic island located between the rear of Buckingham Palace and Hyde Park. It features the Wellington Arch, one part of Green Park situated across Constitution Hill from Buckingham Palace, but now isolated on a traffic island. Beyond Hyde Park Corner from the Palace is the formal entrance to Hyde Park and Apsley House (aka Number One, London), the former home of the Duke of Wellington.
In the episode, "Hyde Park Corner" was the Palace code phrase for the death of King George VI. These code phrases allow the Royal Household to discuss some private matters without the risk of leaks to the press.
The episode shows Tommy Lascelles and his aides bringing the new Queen Elizabeth a full set of mourning clothes. In actuality, the Princess's dresser had already prepared for the possibility that the King might die, and the needed clothing, with the exception of a pair of black gloves, was on the plane. Those were borrowed from the wife of a British dignitary in Kenya. This story only emerged when the lender's godson appeared on a 2012 episode of "Antiques Roadshow" with the gloves.