This is just a quick heads-up highlighting my favorite museum in London, the Imperial War Musem.
Over the decades, I have probably visited this museum four times. Last December, I was actually staying near Waterloo Station and only blocks from the museum visited IWM for four hours on one day and then returned the next day for two hours more.
The museum's focus is the two world wars, though it also has exhibits on Britain's other conflicts in the last 120 years, the Holocaust, espionage, and the meaning of courage. It offers a good cafe and bookstore and is admission-free (though contributions are requested).
For those who like military or 20th century history, it should be a must. It is one of my top sights in London.