I think if you confine the argument to England itself, it's fairly easy to defend that England has been settled by various Germanic tribes which, other than in Cornwall, have supplanted the pre-existing Celtic populations so totally that, around the time of Æthelstan, it was indeed homogenous. Of course, people at the time would consider the concept of lumping the various Germanic tribes and peoples into a "homogenous" identity rather strange, some of them being mortal enemies for centuries. But England in those days was undoubtedly homogenous compared to today. Not sure why that's a virtue in any way, but there it is.