Take a 3D Tour of Buckingham Palace
A number of years ago, Queen Elizabeth II needed an infusion of cash. Fire had severely damaged her beloved Windsor Castle, and repairs to castles really don't come on the cheap. Her solution was to open her royal residence of Buckingham Palace to tours, hoping those interested in an up-close-and-personal view of the palace would help cover the cost of Windsor's repairs.
So far, so good. Tourism at Buckingham Palace has been very strong since the gates were opened. If you'd like to do it, all the pertinent info can be found here.
If you don't have the time, or the cash to pull off a trip to London for the tour, the royal family still has you covered. For the low, low price of free (that's 0.00 British pounds, if I did my math correctly), you can now take a virtual tour of Buckingham Palace without ever leaving the comfort of...well, wherever you're most comfortable.
What's cool is that you can take a virtual-reality tour (assuming you have the VR hardware for your phone), or a 360 degree tour that you can operate yourself.
Since I'm all about saving you some cash: