The short answer is that it wasn't. I partially answered your question here about the Anglicisation of 'England' and how it was a long, slow process over the course of centuries. Although the concept of a unified 'Engla Lond' was mooted by Bede in the early 700s, a single Kingdom of England wouldn't come into being until the 920s, almost five centuries after the Anglo-Saxons first arrived in the British Isles. That's quite a slow process if you consider the lifespan of global super-powers like the USSR (~70 years), the USA (~250 years) or the British Empire (~400 years).

Contrary to what Gildas and Procopius wrote, the Anglo-Saxons didn't arrive, slaughter everyone and take over, but rather integrated themselves over he course of about a century into Romano-British society. When they first arrive, it's as foederati mercenaries, brought in to prop up a Romano-British state struggling to defend itself from Irish, Scottish and Germano-Scandinavian raids without the military and logistical support of the Empire. As the state grows increasingly reliant on the Anglo-Saxons, so they turn on their masters and take advantage of the crumbling Romano-British state to establish their own territorial holdings. To give Gildas some credence, what seems to have occured is the flight of the Romano-British elite, allowing the Anglo-Saxons to establish themselves as the new nobility, as well as intermixing with the local population. It takes over a century for any sizeable Anglo-Saxon presence, whether linguistically, culturally or ethnically, to establish itself on the West Coast, and indeed even as late as the eighth and ninth centuries, the Magonsæte within Mercia may have been Romano-British.

While the multiple kingdoms of the Anglo-Saxons have been reduced to 7 by the ninth century, it's actually probable that the Viking threat catalysed the political union of England in a way that doesn't affect the equally fragmented kingdoms of Wales and Ireland. With a dangerous, foreign enemy on the offensive, the various English kingdoms are forced to put aside their differences and form close alliances. In Northumbria in the 860s, both sides of a civil war unite to fight the 'Great Heathen Army'. Meanwhile the need to extend and reform defences leads to a great expansion of government bureaucracy, centres of authority and physical presence in communities. As the kingdoms of Northumbria and East Anglia fall to the Vikings, they conveniently become part of the 'Danelaw' which can then be liberated and amalgamated into the resurgent West Saxon and Mercian states in the tenth century.