Lets just say in an Alternate reality, Saxons never Invade England, so the Britons keep their Roman style culture. Then when Vikings begin Raiding in the 8th century, how do you think History would change? Here is the Stats:
The Vikings are, well, Vikings. You know what Viking armies and Navies are like.
The Romano British Kingdoms armies are like that of the Late Roman empire. Their main forces would be light-mail wearing warriors, armed with long swords and round shields. Their cavalry would be decadents of Barbarian cavalry divisions from before the fall of Rome, and Archers from Archer Auxiliaries. They would also use a lot of Celtic Mercenaries. The navy would be like of late Rome, with use of Galleys and Triemes.
So, what do you think would happen?