Pros: It's a fantastic city, the political and cultural center of the country, there is a big concentration of libraries, museums, and research institutions in the part of London where I study, it's very diverse, has an impressive amount of visible history, offers many social and employment opportunities, etc.
Cons: Even if you are in halls for your first year, you'll have to travel in subsequent years. Commuting can be a pain at times, there can be unforseen hold ups and delays, and public transportation isn't cheap, although a student Oyster Card helps. Overall it's quite expensive. Secondly, big cities can be tiring and a bit depressing at times. Thirdly, London is quite far flung, and its size can eat up your time if you are going somewhere far from your hall/flat.