When you get to London, you really must make time to go to the Tower of London. The building is magnificent, the history is fascinating, and the views are great. A Beefeater leads the tour from the entry gate to the concluding chapel. The chapel is bittersweet as the bodies of the victims of the Tower are often buried beneath your feet. Depending on your station in life and your gender, you may have been executed just steps outside the chapel (Henry VIII was a particularly vigorous user of the chopping block outside the chapel).
It is not until you get home that you have the time to inspect the photos and notice that it is not a lamp that you have photographed with the Gherkin, it is a security camera.