Although I can't speak from experience, I have many friends who attend universities in London, and I am originally from the London area, and often it seems to be a result of living costs. Shops are, on the whole, a lot more expensive in London than Northern cities, which is where I live now.
Furthermore, they tend to be a lot busier. The sheer amount of people and tourism can understandably become slightly suffocating for students if they're rushing about or need to wait hours to visit one lecturer.