As every tourist that visits London, i've also went to see the Buckingham Palace. The Queen wasn't there. It was sunday, so maybe she went to spend the weekend at the Windsor Castle, her preferred weekend home.
I didn't visit it but as i didn't have a card of the castle yet, i've sent this one to myself.
© Editorial Fisa Escudo de Oro, S. A.
Windsor Castle is the oldest and largest inhabited castle in the world. It has been the family home of British kings and queens for almost 1,000 years. It is an official residence of Her Majesty The Queen, whose standard flies from the Round Tower when she is in residence.
The Queen spends most of her private weekends at Windsor and takes up official residence for a month over Easter (March to April), known as Easter Court. The Queen is also at Windsor for one week each June, when she attends Royal Ascot and the service of the Order of the Garter.
Windsor is still very much a working royal palace. The Castle is used regularly for ceremonial and State occasions. It is here that The Queen often hosts State Visits from overseas monarchs and presidents. St George’s Hall makes a spectacular setting for a State Banquet, when a single table, stretching the length of the hall and seating 160 people, is decorated with porcelain, and silver-gilt from the Royal Collection. - in: http://www.royalcollection.org.uk/visit/windsorcastle/about