If England remains divided, then it is possible that the normans don't invade.
William's 1066 invasion was a result of his claim to the royal throne through the mother of Edward The Confessor. Edward ruled an England unified since the time of Aethelstan, with its unification rooted in Alfred The Great's counteroffensive from Wessex against the vikings in the late 9th century.
Though unification could have happened in a different way, such as during the time of the hegemony of Mercia earlier in the 9th century.
IMO, keeping England south of Hadrian's Wall divided for all of the medieval age is difficult, but doable.