FWIW English as a creole is a pretty hotly debated topic in linguistic circles, and there’s no consensus answer on if it is or isn’t. There’s also some linguists that take issue with linguistic classification of creoles, because many of the assumptions about creolization actually haven’t held up to rigorous scrutiny, and the somewhat “flexible” definition of creole languages (mind the term started very specifically to explain “pidgin languages with native speakers” found primarily in the New World due to mixtures of local and European languages) has left a system where it is difficult to really make heads or tails of what’s what.
English, particularly Middle English, has a pretty weird history in many respects. I think that the importance of Old Norse on Old English is often overlooked, even though it may actually be more significant than the influence of Old French [quick note that the Normans actually spoke a language mutually intelligible with Old French, called Old Norman, but this language had much less influence on English because after a couple generations the Plantagenet dynasty came in and they did not speak Norman but “court” French.]
When you try to define a creole, you typically have to define a substrate and a superstrate…the substrate being a language that an “intrusive” language influences, and the superstrate being the intrusive language. Even languages that aren’t creolized can have substratum and superstratum, for example because of English domination of the development and proliferation of internet terminology, there are many languages that use English words for internet terms, in that context English is being “intrusive” on those languages. However there’s actually active efforts to resist this, for example there are language bodies in, for example, French and German, that attempt to develop and promulgate specific French/German words to stop those intrusive words from creeping in. Their success is mixed. For example the French language academy has tried to promote the word courriel for e-mail, because other words in French for email are seen as not being natively French and to have been influenced by English. Adoption and use of this word in actual French speaking countries is low.
Going back to Old Norse/Old English, Old Norse was a superstratum in the Danelaw, for hundreds of years. Additionally, the “Danes” settled faring communities in the Danelaw and there was regular linguistic interaction between Old Norse speakers and Old English speakers. Believe it or not, interaction between Old English and Old French speakers was much less common after the Norman Conquest than was the interaction of Old Norse/Old English. Why? For two reasons–Old French was spoken by the landed nobility, the heirs to the titles and lands seized by William the Conqueror. Their immediate courts spoke Old French. However the vast majority of their functionaries who implemented their rule were native Anglo-Saxons who spoke Old English, and many of those functionaires did not learn or speak Old French. So how did they work for their bosses? There was already a shared language–Latin. Educated persons (often the clergy filled bureaucratic roles in this era) spoke Latin in both Normandy and England, and it served as a convenient common language between the Norman courts and the functionaries and officials who ran day to day life in the Anglo-Saxon world.
However this obviously wasn’t air gapped. Some of the functionaries would over time pick up some Old French and some would even learn it. Also, the Normans weren’t kept in a bubble totally isolated from the Anglo-Saxons, some large land holders remained Anglo-Saxon, and they would have interactions with the Normans. Words spoken by servants also have an effect of eventually sneaking into the master’s lexicon over time as well. But it was definitely a bit of an odd situation.
However the separation of Old Norse/Old English was much less. Some interesting influences from Old Norse are that many words of “common every day life” the Old Norse version replaced the Old English version. For example the word “egg” is derived from Old Norse, not Old French or Old English. Additionally several important conceptional words, like the word “wrong”, the days of the week, “law”, “give”, “take”, “want”. Also importantly the Old Norse third person plural pronouns replaced the Old English–which is unusual to have happen to a language. The words “they, them, their” come from Old Norse, not Old English or Old French.
My non-linguist take is there was basically a process of partial-creolization and partial-Koneization going on for hundreds of years in England. And oddly still, the grammatical simplification of Middle English versus Old English, was actually happening before the influence of the Norman Conquest, and has happened to other languages without a clear superstratum influencing them, which suggests there is a tendency for many languages to simplify grammar over time even without outside influences.
