Against my better judgement, I will follow it.
Your story has a good beginning, but I don't like OCs unless they are done perfectly. PERFECTLY! So I am a little curious about how this story will go...

Though your prose is a tad too purple for my taste, this is a very intense and promising beginning. The prologue, of course, suggests that you have planned this out to some degree, which means it is definitely worth reading. It is also quite a unique premise, I'd say; usually (which is, I confess, one of the reasons why I find my own similar FF to be becoming boring and uninspired) one would have the child of Nunnally and heir to the throne be incorruptible pure pureness or, at least, very sympathetic. Judging from how you started this, I'd guess there will be a heavy focus on Nunnally's coping with the death of Lelouch, which is always nice. I'm looking forward to being told about the bastard who defiled Nunnally and knocked her up.
It's spelled Britannia, though, with a single "t" and double "n" - an alternative, "Brittannia", was correct at some point, when the word had just been transferred into early Latin from the Greek Πρετταννια, but "Britannia" is the universal spelling in later and contemporary Latin. You will note that the song is called "Rule, Britannia", that the Royal Yacht was HMY Britannia, that the national personification and the Roman province were both spelt Britannia, and so forth. Also, I always found the "vi, li, el ..." appellation system to be inherently ridiculous, but I suppose that's up to you. One more thing: being German, the name "Klaus" sounds incredibly hilarious when used for a "villain". It's considered lame, docile, bourgeois and fifty years out nowadays.
Anyway, this is promising. I shall definitely read on. Do continue soon :)