The British Museum has to be one of the most extraordinary collections of historical artifacts in the world. As many know, the British had issues with Imperialism and tried to stake claim to almost anything foreign they came upon. This meant taking a lot of artifacts and displaying them in this massive house on Russell Street. I really don't encourage you to do the museum in one day, mainly because I think it's impossible! It took me 2 hours just to see the first floor, which holds their most spectacular artifacts in the Egyptian rooms and the Ancient Greek rooms. The King’s Library is quite interesting.
The main reason people go in the masses to the British Museum is to see the Rosetta Stone, the beautiful mummy cases, and basically the most important part of the Parthenon, The whole top of the Parthenon was a wonderful carved story that Lord Elgin basically stole from the Acropolis when he discovered it. I went to Athens to see the acropolis, and I must admit, it was anti-climatic seeing that the better part of the Parthenon was in the British Museum in London, but that's besides the point.
The crowds in these most popular parts of the museum, especially around the Rosetta Stone, never go away, so you just have to bear the hoards of students and tour groups and move on. I also enjoyed The King’s Library, as it has the most fascinating collection of first-edition books and stuffed extinct birds, reptiles, and even fish. There are also many interesting inventions, including different timepieces, a variety of measuring instruments, and much more.
Please don't miss the other parts of the museum. They have a wonderful display of Korean artifacts, things that you cannot find at any museum in America. They also follow the trail of the Silk Road trade industry, which is really quite fascinating. I really encourage you to take the British Museum in snippets. If you're in London for a week, maybe you could visit the museum in 2-hour increments two or three times that week. If you try it in one day, it could be quite overwhelming because there's such a great deal to see.
And after you've seen enough ceramic vases and ancient coins to last you a lifetime, go out and buy those famous sausages topped with grilled onions from the illegal vendors outside of the museum gates, have a seat on the steps of the museum, and take it all in. You really won't be disappointed.