The sheer amount of historically significant objects and manuscripts to be found in London is staggering, and the British Library holds many of these. One of the most impressive exhibits in the Library is the room with four different editions of the Magna Carta. They have an impressive stamp and letter collections, and one of the more interesting letters to be found has stamps on it from both Nazi Germany and Stalin's Russia.
Here's an interesting video that will give you some more insight on what exactly can be found here at the British Library. Enjoy!