Hey, this is a fairly common question, which I most recently answered here.

The tl;dr for people who don't want to click through to other threads is that the concept of a single polity consisting of the unified kingdoms of the North German peoples in Britain was first conceived in literature by Bede in the 8th Century in Wearmouth-Jarrow, when Anglian Northumbria was the dominant political power in Britain, its closest rival was Anglian Mercia and its predecessor had been East Anglia. In this landscape it was only conceivable that a future united England would be Anglian in nature. The concept of an Anglalond stuck in political consciousness.

By the tenth century, when unification was really on the cards, Mercia had had made a significant contribution to the consolidation and expansion of England, one which has subsequently been overwritten by an extremely effective West Saxon PR machine. Stafford and Molyneaux (and I) posit that the continued success of a unified England was balanced on the continued good will of the (ethnically Anglian) Mercians, as well as the willingness of the Anglians of Northumbria and East Anglia to forget centuries of independence and political supremacy to cooperate in a post-Danelaw unity. Æthelstan had witnessed the politics of securing this cooperation at the court of his aunt Æthelflæd in Mercia, and may well have adopted the concept of an Anglalond in part to assuage the concerns of the Anglians that their political and cultural independence would be lost in a Saxon-dominated political entity.