Mansion House is the official residence of the Lord Mayor of London. It is a Grade I listed building. It is used for some of the City of London's official functions, including two annual white tie dinners. At the Easter banquet, the main speaker is the Foreign Secretary, who then receives a reply from the Dean of the Diplomatic Corps, i.e. the longest-serving ambassador. In early June, it is the turn of the Chancellor of the Exchequer to give his "Mansion House Speech" about the state of the British economy. Wikipedia, Mansion House

Hughes
Dr. Gauntlett was looking forward to the honour of meeting His Royal Highness the Duke of Edinburgh at the Mansion House, on February 22nd, 1876, regarding the formation of a new English College of Music, and the following notes were evidently the germ of what was passing in his mind on the views which he hoped to express. The reform and elevation of sacred music had been his life-long aim, and he was hoping, under royal sanction, to attain a wider hearing for his opinions. Providence interrupted this plan by his sudden removal from the world the day before the meeting. [Harmonies of Tones and Colours, Fragments from the Last Note-book, page 50]