With the Romano-British and the Normans, the South East seems to have been the seat of power, I suspect because of its closeness to the continent. Yet with the Anglo-Saxons, despite their having arrived in England in the South East, it seems it was the South West that became dominant. For example, West Saxon was the prestige Old English dialect, and King Athelstan of Wessex united England.
What was different then that led to Wessex being dominant?
Mostly because that's where most of the action took place. If you look at grave-good distributions, the large majority of them fall in Wessex/Mercia (the other main area being Northumbria), indicating a reasonably strong trade network, helped by the centrality of the Thames and linked rivers. Wessex also sits by virtue of geography, smack in the middle of everybody, which allows expansion in any direction if the opportunity arises. The south also has the largest number of religious houses (or rather, the largest number of continuously existing houses). This is helpful for language and stability.
Yorke suggests that the Northumbrian and Mercian houses disappear in the 8th century because they became less economically and militarily successful, while West Saxon kings were better annexing land from monasteries and maximising their efficiency. She does finish by saying that it's difficult to explain why Wessex survived. She attributes it mostly to geography and trade, and a bit of luck and happenstance. Northumbria declines because of an over-reliance on overlordships, and giving away too much land to the church. Mercia in part, disappears because of the failure to produce male heirs. The last and biggest reason perhaps, might be the Vikings and the Great Heathen Army (see the map) who took on Wessex last, essentially running out of steam.
http://www.historytoday.com/russ-foster/alfred-great
http://www.reformationsa.org/index.php/history/94-king-alfred-the-great
Good leadership from a dynasty of wise and courageous Kings, the pinnacle of which was the genius and saintliness of King Alfred the Great (pbuh) whom Winston Churchill described as the greatest Englishman who ever lived. He was proof of the Shakespearean line that some men are born great and it was a blessing of fate that he was also born to be King. It's not very often that an individual can make a huge impact on a nation's history and it's even rarer that it be for the good, but in this case it happened, it's a story book narrative that happens to be true.