@Abdominalz
That's a good question : we have to take in account the nature of Anglo-Saxon kingdoms, at least up to the VIIIth, or why there was an unification trend.
See, the patchwork of petty kingdoms of various size and borders is explainable by their tribal nature : contrary to what happened in Romania after the Vth, the various Germanic people found little post-imperial infrastructures to really break-out their traditional frames, and the relative lack of contact with late imperial world (at the contrary of, say, Franks, Burgunds, Herules, Goths, etc.) certainly didn't that helped.
You have the Saxon establishments in coastal southern Britain, of course, but it was relatively unrelated to later Germanic settlement which is more a by-product of troubles in northern Germania with the fall of the roman state in the west. You'd notice, tough, that Wessex seemed to have been slightly more firmly structurated than its neighbouring kingdoms.
Anyway, tribal kingdoms of the early Anglo-Saxon periods could be considered as what Colin Renfrew (at this point, I'll heavily borrow on Richard Hodges) devised as chiefdoms, meaning an early state structure which takes place in an ensemble of similar early polities (with similar sizes and populations) where ranking and social differences are polarized on a chief and its lineage, build on a center of power, where the social role of economy is principally based on redistribution.
From time to time, some chiefs are able to draw on mobilisating features (which are usually drawn from the necessity to "fund" an elite/army, etc.) to increase their power, eventually drawing on neighbouring communities, which increase mobilising features, etc. It forms an unstable, but dynamic and structurating type of chiefdom that can be labelled as complex or cyclical. Cyclical because while it unify at least part of the chiefdom ensembles, it tends to crumble on its own weight, due to the lack of infrastructures strong enough to support it more than some generations. Cyclical chiefdoms tends to keep the frame of tribal structures relatively intact while ruling over these.
But, even if crumbling, it did let more frames for the next candidate to live on, and even if it does fail again, the process would continue before stabilizing more or less definitely.
Of course, it's a model and not a procedural guidebook, but it does explain well the geopolitical and social evolution of Anglo-Saxon polities : tribal division, rise of Northumbria, tribal division, rise of Mercia, division, etc. Sort of dialectical process, you might say.
Back to early Anglo-Saxon England : the first tentatives at unity were fairly short-lived, and usually involved the southern kingdoms that kept more of sub-Roman structures (as well as a strong influence from Francia for some of these, especially Kent but as well Wessex), but eventually, more clear imperii appeared : Northumbria (which, even not becoming the unifier of England, did already managed to unify its immediate neighbouring, I'll get back to this), then Mercia, then Wessex. Causes of rise and declines weren't, of course, only a matter of inner situation but as well outer (Frankish influence as said, conflicts between kingdoms, and of course raids and invasions, or failed campaigns as for Ecgfrith).
So, we have something to consider there : unification trends in a metapolitical ensemble answering to same socio-cultural frames is likely to happen without massive interruption, but it doesn't have to be complete, as partial unifications as it happened to Northumbria seems to have worked out.
On this regard, we must find a reason to block the unification of England, while not keeping the initial stage of a whole blur ensemble of tribal kingdoms. What really managed to blockade this evolution IOTL was the Scandinavian takeover : it remained fairly well into the frames of middle Anglo-Saxon kingdoms (Northumbria, East-Anglia, a bit less Mercia), and I think the best answer to your OP would be to perpetuate, for a while longer, the division between Anglo-Saxon and Anglo-Scandinavian Englands.
It's hard to say how exactly :
more successful Great Heathen Army that would swallow up the entiere Mercia? That could be an answer, altough you might have a thin line to respect between pushing back Wessex, and beating it up at its own game (even if I find this unlikely).
I think you'd need something more, possibly a weaker Wessex in the wake of Scandinavian takeover (a stronger Mercia, stronger sub-kingdoms in the east).
Maybe a Wessex defeat at Higston Downs?
Admitting we did set up an Anglo-Dane Jorvik/Mercia ensemble, and that Wessex is busy elsewhere (possibly focusing first on East-Anglia?), you do have a separate ensemble, or rather two separate ensembles : one Anglo-Saxon, one Anglo-Dane that could with enough luck survive as such.
The result would be a more Late Carolingian looking Wessex, IMO : more busy with continental business it alreadyw as (would it be only because it would depend more on continental happenance that it did IOTL), and a more North Sea focused Anglo-Scandinavian ensemble that while might be busy a bit with the Carolingian world, wouldn't be at all dependent from it and more from structuration from northern Britain and Scandinavian ensemble in northern Europe.
