A rough timeline with a POD likely around 450-500 CE:
-The Franks manage to conquer Northern France but don't unify and you have Salian Franks and Ripurian Franks. They don't conquer the Alemanni or Burgundy either, we have a divided NW Europe.
-The Frisian push southwards and expel the Salian Franks out of Flanders in the early 6th century. By 600 Frisians control all of the modern low Frankish speaking areas, lower Rhine and the Ems basin in Germany.
-The Anglo-Saxons conquer England a bit faster, maybe by avoiding losing against the Britons in the battle of Badon or whatever historical counterpart of this battle there was. By 600 CE all of England is under stable Germanic control.
-Let's postulate that a different strain of Christianity, maybe some Arian-like heresy, ends up taking over Germanic speaking areas, the Frisians slowly convert to this faith and spread it around the North Sea, let's postulate also they translate the Bible in North Sea Germanic, thus giving a base to maintain a prestige dialect for the tongues spoken from Jutland to Wales during this period.
-The Frisian kingdom eventually ends up conquering the Saxons to their East and control a large kingdom Rugen to Calais by 800 CE.
-A Frisian backed Anglian kingdom ends up defeating a Frankish backed South-Western Saxon kingdom around the 7th-8th century and ends up with a situation analogous to the Mercian hegemony during the OTL 8th century.
-Alt-Norse entering the scene during the late 8th century would be more marginalized to raiding and settling the North-West of the British isles and the Baltic. Instead of being pagan they start already as an hybrid between Arian Christian and Norse Pagans and their religious difference from the more Chalcedonian Celts is maintained. They eventually Norsify all of the Scottish Highlands, Isle of Man and some enclaves in Ireland but their language eventually drifts towards the Frisian-like prestige variant as the Anglic kingdom to their south subjugates them in both violent and non violent ways. The Norse also settled Iceland and Greenland during this time.
-A more united Anglian kingdom in the 8th century would be eyeing both to Ireland France to the south, with the Norse going around the Anglos use them as mercenaries and allies to raid and attack Breton and Frankish isles, coasts and rivers. Eventually you have a Anglo-Norse kingdom in Britanny and Western Normandy by 900 CE.
-The Frisian kingdom during this period would end up slowly englobing areas upstream from the Rhine, the Weser and the Elbe, conquring Thuringia and defeating pagan Slavs around OTL Upper Saxony and Brandenburg. North Sea Germanic becomes the primary language north of the Thuringia forest and north of the Main river and the Frisians would control all of that region by around 1000 CE.
In this timeline I really define "English" broadly because frankly before 1066 and especially before the Frankish conquest of Frisia and Saxony North Sea Germanic was far more cohesive.
English, despite many people wishfully thinking the contrary, has very little Celtic influence and if we postulate a North Sea Germanic language being the prestige dialect instead of Latin it would also have far less Romance influence, by 1000 CE we would see this rather cohesive language area dominating from the Oder to Britanny, all deriving from Frisians and the Anglo-Saxons people that came from the Jutland-to-Frisia coast starting from the 5th century.
We also had multiple events that made this language more cohesive and also hegemonic in its influence over Norse people, thus also maintaining unity beyond the rather arbitrary West/North Germanic lines that exist IOTL given the different circumstances.