My best guesses would be the Danes/Vikings/Normans.
And maybe even the Germanic tribes, the Germans being more of the tribes type of approach they can represent all the different tribes of that era where England was in it’s infancy.
My personal fav longshot is the Rome. My reasoning behind this is that we see they very beginning of the English and even the Mongols, now this version of Rome could be early game focused mirroring their historical standing in relation to the events. Or we could see a late version of Rome, Rome could be based on the final couple of decades of the Roman empire just before the Huns descend on them.
Here is a outsider to consider, The Goths, they originate from somewhere in France, one of the only few people that Rome couldn’t enslave, they instead granted the goths their own piece of land but under Roman law, the Goths could be a interesting choice.