Key Points:
London is one of the best travel destinations in the world, and is also an important part of travelling around the world.
Nomadic Matt has provided a pretty good travel guide for London. There are also some itineraries from wikiVoyage, which could be a good one-day trip as well.
The primary things to do in London include: British Museum (1 day), Science Museum (1 day), Trafalgar Square and Tower of London (1 day), Stonehenge (1 day), Oxford (1 day), Cambridge (1 day), London Wall (1 day), South Bank (1 day).
According to Wikipedia, "London is the capital and largest city of England and the United Kingdom. It stands on the River Thames in south-east England at the head of a 50-mile (80 km) estuary down to the North Sea, and has been a major settlement for two millennia. Since the 19th century, the name "London" has also referred to the metropolis around this core, historically split between the counties of Middlesex, Essex, Surrey, Kent, and Hertfordshire, which largely comprises Greater London, governed by the Greater London Authority."
Here we quote the best way to travel in London provided by Nomadic Matt, a New York Times best-selling author of How to Travel the World on $50 a Day and Ten Years a Nomad, as well as the founder of his website.
There are also some itineraries from wikiVoyage that you can try as well: