NOTE: I do not own Code Geass. I created this timeline primarily because one of my future Code Geass fanfics will revolve around this timeline. Enjoy, and please review if you could.
Ch. 1: The Legendary Beginnings to the Golden Age of the Tudors
55 BC-54 BC. - The Celtic tribes of the British Isles unite and make Eowyn Chifetain after repelling Julius Caesar's invasion of the British Isles
43 AD - Rome successfully invades Britannia reducing Chieftain Llewlyn's territory to a fraction of its size
5th-6th Century - Anglo-Saxons invaders drive Chieftain Wynne to flee to modern day South Wales
1066 AD - Chieftain Daman (note: Daman is the first non-legendary Britannian ruler. The authenticity of those before him is still questioned) allies himself with William II, Duke of Normandy against Harold Godwinson. As a result of victory, William makes Daman Duke of Britannia, therefore making Daman one of the few native Brittons to keep power.
1282 AD - The Duchy of Britannia remains intact despite English takeover of Northern Wales
1576 AD - Elizabeth I gives birth to Prince Henry despite not being married. Nevertheless, Parliament and the nobility recognize Henry as heir in order to preserve the Tudor Dynasty. Potential father Carl, Duke of Britannia therefore gains greater influence in the royal court
1603 AD - Elizabeth I dies and 27 year old Henry is crowned Henry IX. Since the Scottish Stuarts don't recognize Henry as legitimate, Scotland protests and demands King James VI of Scotland to succeed to the throne. To appease Scotland, Henry IX promises to have his daughter and heir Elizabeth (born 1600 AD) marry Henry Frederick, Prince of Scotland. Henry IX also appeases the English nobility by promising that the future heirs of Elizabeth would use the name Tudor instead of Stuart
1603 AD-1637 AD - Henry IX colonizes the New World
1625 AD-1651 AD - Henry I of Scotland rules
1637 AD-1682 AD - Elizabeth II reigns while expansion in the New World continues
1682 AD - Prince James (born 1632), already James VII of Scotland since 1651, is crowned James I of England. King James VII and I immediately unites both England and Scotland into Great Britain through royal decree
1691 AD - 38 year old James VIII and II is enthroned
1709 AD - 18 year old George I is enthroned. Meanwhile Britain, with Louis, Duke of Britannia leading the troops, competes with the other great powers over colonial policies and territory
1752 AD - 23 year old Henry X is enthroned
1775 AD-1781 AD - Washington's Rebellion erupts in Britain's North American colonies
1777 AD - Benjamin Franklin is bribed by Edward, Duke of Britannia to defect to Britain's side in exchange for noble titles. Benjamin Franklin therefore becomes an Earl and flees back to Britain. The exact details behind this arrangement are still not know to this day
1781 AD - Washington's Rebellion crushed at the Battle of Yorktown
1789 AD - French Revolution erupts. French Republican forces start spreading across the European continent.
1794 AD - 65 year old Henry X dies and Edward VII is crowned King of Great Britain
1796 AD - Edward VII dies from natural causes. His sister Elizabeth succeeds him as Elizabeth III
Author's Note- The names of all the Britannian Chieftains mentioned (except for Eowyn) are all based on actual Celtic names. Llewyln means "leader" while Wynne means "fair" in the Celtic language. Daman is Celtic for demon, which I thought was perfect since it kind of connects with Lelouch the Demon Emperor.