I think the crux of this issue is that in the eleventh and early twelfth centuries, England and Normandy were seen as distinct territories that happened to usually have the same ruler. Just because a man became King of England did not make him automatically Duke of Normandy – or vice-versa, as in the case of William I’s sons, Robert Curthose and Henry I. Uniting them into a single polity would have been impossible, and potentially undesirable, given the clear distinctions between their administration and legal traditions.

Why would it have been undesirable? Legitimacy of kingship was a vital issue in medieval societies. The authority of the Norman kings to rule England was derived from Edward the Confessor, cemented by the rituals of English coronation and blessed by the Church. Upon becoming king, the new monarch would promise to uphold the traditions of English kingship, dating back to the time of Edward. Writing the kingdom out of existence would have also meant losing their legitimacy. Don’t forget that even after the Conquest, Normandy itself remained a nominal fiefdom of the French crown – being King of England did not confer any special privileges in this regard.

Additionally, it is apparent that the Norman kings saw the English system of government as a viable one. Although they had a significant impact architecturally (especially through the foundation of castles and new churches), the Normans showed little desire to rip up the secular administrative tools they found in England. Sheriffs and earldoms both survived the transition, and though these offices might now be held by Frenchmen rather than English, the Conqueror and his successors innovated rather than expunged their use.

This question also comes down to how the Normans in England viewed themselves and their new subjects. Looking at the charters issued by Norman rulers in the twelfth century, we can see that they did not officially consider Normandy as a separate nation, occupied by a special people. Instead, they address their proclamations to their lieges “French and English in England and Normandy” – not “Norman”. The Normans ruled over a vast domain and most of their subjects were undoubtedly English – by language and by culture. Artificially trying to overcome this reality by renaming the kingdom would have been a pointless waste of energy, damaging to their prestige and a huge incitement to the already unruly English majority.

Sources: M. Morris, The Norman Conquest, (London, 2012)

D. Matthew, King Stephen, (London, 2002)

M. Chibnall, “The Charters of Empress Matilda” in ‘Law and Government in Medieval England and Normandy’, (Cambridge, 1994)