Solid post Harald. I believe that the two most pertinent questions you asked in relation to naming the First King of all England was
"What proportion of modern-day England do we think an Anglo-Saxon King had to have control over for us to call him the King of England? And for how long?"


Edward the Elder was King of all the English south of the Humber river. He received the "submission" of all the northern kings in 920, thus making him overlord of their areas, albeit only in name.
Although there were some signs of discontent in Mercia, Athelstan was certainly the undisputed king of the Sutangli, or Southern English. However, Athelstan enjoyed a relationship with Northumbria that his father never had the power to do. So it is quite compelling to name Athelstan the first King of all England.

Now Harald's question is relative. How long did one have to rule this land in order to be King? Egbert was bretwalda, but these lands eventually fell from his control.

Troubles plagued the descendants of these late Saxon kings, and then the Vikings came back to usher in a new age of invasion. I think that it comes down to this. If there were no losses of accquired land, nor any string of events that threw out the dominance of the West Saxon dynasty from these lands, then Edward the Elder must be named the first King of England. This is largely a nominal title, but still has relevant and important historical implicaitons attached to it. Athelstan was mighty in battle, but he really just continued the work of his father.