A new development in the United Kingdom (which like many other countries is strapped for cash) is to allow extensive tours of Buckingham Palace when the Queen and family are away during the summer. My wife and I with friends from Dorset, England spent the better part of the day touring Buckhingham Palace. It was a truly wonderful experience. Although the price of admission is quite pricey (about $50) you get a lot for your money. For the ambassador admission you get a timed admission and earlier in the day is my recommendation. One starts with a tour of the 100 room palace filled with art work by famous painters and hundreds of historic items. The rooms, ballrooms and theaters are truly fascinating and their is also a display that provides a montage of historic photos that document the amazing life of Queen Elizabeth II and her family. Fergie is noticeably absent.
After the tour one can eat at the terrace restaurant, but the lines are long and seating difficult if you are around the lunch hour. You can skip the lines and go to the gift shop and then buy an ice cream as you follow a pathway to see the extensive grounds in the heart of London.
Then after a long walk back you can tour the royal carriage house to see incredibly ornate carriages and more historic paintings. Finally there is the art gallery with more of the Royal collection of paintings. There are other admission options, but the full menu of activities that take up at least 4 hours is highly recommended.