It depends. After about 890 Wessex and western Mercia were beginning to merge into a single England so screwing Wessex after 900 might just mean replacing the ruling dynasty with a Mercian one.
I posit a victory for Dublin, Scotland, and Strathclyde at the Battle of Brunanburh (927) as a PoD, but i don't know if that would be enough to totally screw Wessex and/or Mercia.
It depends. After about 890 Wessex and western Mercia were beginning to merge into a single England so screwing Wessex after 900 might just mean replacing the ruling dynasty with a Mercian one.
The reason why I put it after 900 was that I thought it would be interesting if Southern Britain was divided enough to make it easier to conquer and keep at least parts of it for one of the rising Scandinavian monarchs. Alternatively, maybe a Welsh kingdom could start a "reconquista"?