Board Thread:Questions and Answers/@comment-24411698-20140318010448/@comment-11596933-20140531231137

There is like a mini animation with Lelouch and Suzaku that seems to describe the history of Britannia. Not sure if it is canon or not but here is the link: http://www.youtube.com/watch?v=dJinX-eOE-8

If you don't want to click the link just search "Code Geass - Origin of Britannia Part 1" and you will find it. Hope that helps!