Winfield House is a mansion set in 12 acres (49,000 m²) of grounds in Regent's Park, London, England - the largest private garden in or close to central London after that of Buckingham Palace. Since 1955, it has been the official ambassadorial residence of the United States Ambassador to the United Kingdom.
The Wikipedia page is [1], and the US Embassy & Consulates in the UK page is [2]