Hello, I answered this question here. The tl;dr is that the concept of a unified polity in what is now England first appears in the 8th Century, conceptualised by Bede. Bede is a Northumbrian writing at a time when the Anglian-identifying Northumbria is the dominant political power in England, and as such it follows that he envisions this Anglalond to be centred around Anglian supremacy. It may just be that this is a name and concept which sticks. It is Bede, of course, who delineates the various kingdoms of Angles, Saxons and Jutes along ethnic lines whether or not that was actually the case on the ground. To some extent, the idea was picked up; Welsh contemporary chronicles do distinguish between the Anglii of Mercia and the Saxonum of Wessex, and as late as the C10th, coins of Æthelstan's Rex Tot. Brit. series minted in Derby are instead marked Rex Saxonum.

Tradition may be underlined by political necessity. The 'unification' of England in the early C10th is, despite West Saxon PR, still very dependent on the continuing good will of the Mercians in particular, but also the East Anglians and Northumbrians. Pauline Stafford in particular hypothesises that Anglalond may have come to the fore once more as a sort of political concession to other kingdoms worried about West Saxon dominance.

Much more can be said about the emerging identities in Early Medieval Britain; but you might be interested in this

Basically, names such as Angles, Saxons or Jutes seem to have been less ethnic denominators that were translated across the North Sea than names that (re)-emerged in the VIth century to name and explain the new political realities and distinctions.

Angli, for instance, appears first in Byzantine and Merovingian courts and possibly out of reference to Tacitus' De Germania, to name peoples in relations with Franks and christianized Germanic societies in Britain. A name appropriated by Anglo-Saxons themselves, possibly out of confusion (or trough association or pun?) to angulus, desolation, before getting associated with a regional, Northumbrian, identity.