There are three editions of the British National Corpus, the original first edition (1994), the BNC World Edition (2001), and BNC XML edition (2007).
This documentation refers to the World Edition, though nearly everything in it will apply to the original edition too.
A good source of information is Guy Aston & Lou Burnard, 1998, The BNC Handbook, Edinburgh: Edinburgh University Press. Dave Lee (http://devoted.to/corpora) has lots of useful information too.
This guide does not attempt to represent all the facts about the BNC in detail since the BNC site itself and the Aston & Burnard book contain so much definitive information. Instead it concentrates on what you can do with the BNC using WordSmith Tools.