When Abel Gance's started his work on Napoleon, he envisioned doing an extended series of several films to cover his life. Due to the sheer time and cost, he did only one film. The London showing will show nearly five and half hours of the currently restored version by Kevin Brownlow.
The film doesn't cover all of Napoleon's life, only part of it. This PBS documentary on Napoleon is a great introduction to him. Part one roughly covers the years presented in Gance's film. (Personally, I found it best to watch the full documentary, as a refresher on his life.)
Just remember, Gance's film is his own version of Napoleon life, not a documentary. If you have watched other Gance's work, you already have a fairly good idea of his storytelling style. An excellent pre-film book to read is Kevin Brownlow's NAPOLEON. It goes into detail on how the film was made. It could increase your appreciation on the creating of the production, and a great edition to your silent film library, whether you are going to the film or not.
For more, here is a series of my post about NAPOLEON.
Also, for those who like reading reviews, before seeing a film, this thread on Nitrate has varies reviews about the US showing and others. I post the thread link, for those who maybe interested. As I mentioned on my blog before, if you are planning on seeing it, I think it's best to see a film through your own eyes and experiences.
Update at this posting: For the Napoleon showing at the Royal Festival Hall on November 30th, about 25 seats left.