I think that the Norman conquest can be labelled A turning point in its history but not THE turning point.

The Norman conquest can be considered one of many turning points in British history insofar as it brought changes to the power structure, government, language, and society. But I think the say that this is the turning point is to imply a lot of things that necessarily shouldn't be. Saying that it is the turning point is to subscribe to a certain historical narrative that surmises that the Norman conquest was the basis of a great many things that happened in the subsequent millennium. Can we really say that without the Normans Britain would never have risen to power centuries later? We can't if only because there are an infinite number of variables that make engaging in counter-factuals over such a broad scope an exercise in futility.

Turning points are a tricky thing that are the subject of many heated debates. Even topics of lesser scope then the history of a nation and its people face intense debate. Scholars of WWII, for example, often go back and forth over the decisive turning point of the war. Was it Pearl Harbor, Stalingrad, Midway? The answer to that will vary with how one views the topic and to a degree on which narrative a historian is viewing the war through, or looking to promote.

Ultimately my answer is not one of debating the historical merits of the Norman conquest but one of arguing that reducing an infinitely complex and rich history down to a singular moment is difficult, tricky, and can serve to simplify that which shouldn't be.