The British Isles may not have remained independent from the Romans or if current royal family trace there lenaige back to Eowyn . It is not clear if the British Isles were also able to fight off the invasions by Germanic barbarians during the Dark Ages, i.e. if “Britannians” are descendants of “Anglo-Saxons”. In Our Timeline, the Germanic invasions pushed Celtic peoples to the mountainous fringes of the British isles, i.e. Wales and Scotland; thus if the Saxons never invaded in the Code Geass timeline, “Britannians” are ethnically Celtic. It is also not clear what effect the Viking invasions may have had, or if the conquest of the British Isles by the Normans under William the Conqueror actually happened.
Early Modern era:
The exact details of Britannia’s growth through the 17th century and the Age of Revolutions are only heard in bits and pieces.
In Our Timeline, Queen Elizabeth I died childless as the last of the Tudor Dynasty. In the Code Geass timeline, while Elizabeth I remained unmarried, she bore a son whose father was never publicly identified. Potential fathers were Sir Robert Dudley, 1st Earl of Leicester; Sir Robert Devereux, 2nd Earl of Essex; and Sir Carl, Duke of Britannia — these men would use this knowledge to gain influence and power. Whatever his parentage, Elizabeth I’s son ascended the throne under the regnal name “Henry IX”, upon the death of his mother in 1603. Hentry IX’s ascension was said to mark the beginning of “The Golden Age of the Tudor dynasty.”
Britannia’s history started to drastically diverge from Our Timeline during the Age of Revolutions, starting with the failure of the American Revolution in the 1770s. The American War of Independence, officially known in the Code Geass timeline as “Washington’s Rebellion”, did not succeed in overthrowing Britannian rule because the Duke of Britannia bribed off Benjamin Franklin, awarding him the title of earl. Washington’s Continental Army suffered a crippling defeat at the Siege of Yorktown, and Britannia’s control of its North American colonies was secured.
Exactly what the global possessions of “Britannia” were at this time, relative to Our Timeline’s “British Empire”, is not clear. It is not certain if Britannia ever colonized the Indian Subcontinent (because China was expanding into it), or if Britannia was ever an active colonial power in Africa. Heading into the Age of Revolution, Britannia’s most imporant overseas possessions seem to have been in North America.
The Age of Revolution:
As the Age of Revolution reached its peak, many of the old monarchies in Europe were overthrown, but Henry X of Britannia retained an absolute monarchy. The greatest of these revolutions was the French Revolution, which eventually produced Napoleon Bonaparte. The Napoleonic Wars were the single great formative event which resulted in the creation of the “Holy Empire of Britannia”.
Napoleon decisively won the Battle of Trafalgar, and proceeded to successfully invade the British Isles, occupying London (in our timeline, the French invasion fleet suffered a crippling defeat at Trafalgar to British Admiral Nelson). Napoleon’s invasion of the British Isles advanced to the point that in 1807, Britannian Queen Elizabeth III had retreated to Edinburgh, where a local revolutionary militia arrested her and forced her to abdicate, ending the monarchy over the British Isles. This event became known as the “Humiliation of Edinburgh”.
However, Sir Ricardo von Britannia, Duke of Britannia, and his friend and subordinate, Sir Richard Hector, Knight of One, led Elizabeth III and her loyalist followers on a retreat to Britannia’s colonies in North America, establishing a capital on the east coast. In 1813, the childless Elizabeth III was on her deathbed, and so she nominated Sir Ricardo von Britannia as her successor upon her death. At this time, the new Britannian calendar system “Ascension Throne Britannia” (A.T.B.), dating back to the first Celtic king, is retroactively established. Although Napoleon had successfully conquered the British Isles, in conjunction with popular uprisings against the monarchy, his rule over Europe ran into popular discontent, culminating in his defeat at the Battle of Waterloo in 1821. Napoleon died on his way back to France from Waterloo, and it is rumored that he was poisoned by agents acting in accordance with Elizabeth III’s will. The result was that popular uprisings overthrew Napoleon’s imperial ambitions, and reformed the new Europe-spanning empire to keep it in line with the democratic ideals of the Age of Revolution. This would later grow into the “Euro Universe”, expanding to control all of Europe, Africa, and Russia, and for centuries would remain the bitter enemy of Elizabeth III’s followers that fled to North America.
Development and Expansion:
By the end of the Napoleonic Wars, “Britannians” were essentially the defeated remnants of the absolutist aristocracy of the British Isles who had retreated in disgrace to their colonial holdings in North America, after completely losing their original homeland. Even before the Napoleonic Wars, the Britannian aristocracy had utterly rejected all of the democratic reforms of the Enlightenment and Age of Revolution, embracing an old-style absolute monarchy dating back to the Tudor era. Their humiliating defeat and exile only further embittered the Britannians, resulting in them fully embracing a national ethos of revanchism. The modern “Britannian nation”, while geographically centered in North America like Our Timeline’s United States, is directly analagous to neither “Britain” nor “America”. The Britannians are an odd mix of Tories from North America (the pro-British colonials in the War of Independence) and imperial British aristocrats.
This small population of “Britannian” aristocrats, having retreated to North America, had reached a critical low point, and had a very small core population. They were determined, however, to rebuild their powerbase now in North America, and began an aggressive campaign of conquest. Having been pushed to the brink of extinction, the Britannians now saw international politics as a fight for survival, and they rapidly rebuilt themselves into a world-class power by dominating other territories and peoples in North America, and over the decades, South America as well. Each new conquered region was designated an “Area” and its indigenous popluation “Numbers” (named after their Area, i.e. people living in Area 11 are “Elevens”). Because the original core Britannian population was so small, they developed a very stratified and highly classist social hierarchy, in which an extremely small minority of “pureblood” Britannians (descendants of the original British aristocrats that fled to North America), ruled over “honorary Britannians” (who held a mid-level social position), and at the very bottom, the “Numbers” — conquered peoples who lived in segregated-like, near-slavery conditions.
During the second half of the 1800s, Charles Darwin proposed the Theory of Evolution, which was later appropriated for the philosophy of Social Darwinism: the idea that conflict and competition promote growth, and essentially, “might makes right” — those who conquer and subjugate others are justified in doing so, because it promotes their own survival. For the Britannians, who had struggled to claw their way back from the brink of extinction and over decades turned themselves into a world power again, Social Darwinism was the ultimate justification for their campaign for survival, conquest, and ultimately revenge.
The “Holy Empire of Britannia” utterly embraced Social Darwinism as not only their national ethos and mentality, but something akin to a national religion of sorts. Darwin became refered to as “Saint Darwin” etc. What was once an oppression of conquered peoples as a matter of survival and necessity became a systematically applied set of negative virtues. Essentially “reforming itself along Prussian lines”, Britannia’s entire society became dedicated to the military effort and further conquests.
By the time of the (unspecified) world wars in the mid-twentieth century, barely a century after a handful of British aristocrats had been driven to their North American colonial holdouts, the Holy Empire of Britannia had become one of the three global superpowers (the other two being their bitter enemy the Euro Universe, as well as the Chinese Federation).
By the early 21st century, from their homeland in North America, Britannia controlled both American continents, as well as (depending on if different versions from the manga do not contradict the main storyline) New Zealand, Hawaii, and the Philippines.