Best POD to butterfly a united England

So, ensure that there will be no England. I am thinking here of weakening Wessex and the other Anglosaxon kingdoms to the point that no Anglosaxon kingdom is able to unite Anglosaxon Britain. What is the latest POD that can achieve this? I do not necessarily assume that there will be a united Daneland instead, at least not in the short run.
 
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