IMO this is really the key. With only four, reasonably big, kingdoms being reasonably viable the area, almost inevitably IMO, entered a dynamic of shifting hegemony, with Northumbria, Mercia and Wessex all having their turn, and there is no reason per say East Anglia couldn't have had theirs at some point. Someone was bound to ''clinch it'' at some point...
You need the fracture future England significantly more then it was to keep it divide indefinitely, or alternatively have some kind of POD far enough in the past to make the constituent kingdoms different enough for any unification to be ephemeral, but there was significant differences in OTL as it was and it wasn't enough...