1. This was only a few centuries after Roman occupation ended. As we know Roman influence on culture lingers but not evenly so I imagine even in the 10th century, you would find some people and areas where there was no Roman influence or culture at all and others far more recognizable as Roman influenced;
2. Great Britain was obviously divided by Hadrian's Wall culturally. There were various Celtic culture in Great Britain and Ireland prior to Roman occupation. Some of these are extinct now. Remnants of others have survived (ev Gaelic in Ireland). Celtic was subsumed by Roman culture in England to a large degree but not at all in Scotland; and
3. Viking invasions. You have to remember that Viking influence extended all the way to France with Rollo becoming the ruler of Normandy a century before. Raids extended to Paris. And of course you had the Viking ruler in Northumbria.
So I always assumed that the idea of a homoogenous English "culture" was a product of the unification of England politically.