
What if Roman Britannia somehow remained in the empire and became the last province of The Western Roman Empire during its collapse, then Rome's elite moved there and made Londonium the capital?
Let's just say The British Isles as a whole don't get attacked by anyone, and the territory remains a part of The Western Roman Empire as a largely forgotten, remote island up until the Fall of Rome. Perhaps the Romans in Rome see they are fighting a losing battle after the first Visigoth raid, and abandon the city for Londonium by boat, vowing to amass strength to later reclaim territories from the Germanic barbarians. How might this have changed things?