London, the capital city of England, holds my heart because it was the first international city I visited for work. It is a great "first experience" city for Americans because they also speak English (although with an accent and some turns of phrase that can sometimes be difficult to comprehend at first!)
Many itineraries will have you staying in London for a couple of days and then moving further afield. If you have a limited amount of time this will work, but I suggest spending a week soaking up all the city has to offer prior to moving on. It is a very walkable city, and the London Underground, "The Tube", as well as the touristy Hop-on-Hop-off buses offer very convenient transport.
If you are a lover of history, there is no better place to be than London. From the Roman Ruins to WWII and the Monarchy, you will have more than enough experiences to enhance your knowledge!