‘Anglo-Saxons’ is the name applied to the groups of people who arrived in Britain from northern Germany and Denmark in the 400s. They spoke various dialects of a language related to German that we now call Old English.
In eastern Britain Anglo-Saxons took land from the native British people or settled down to farm alongside them. It is not clear how much intermarriage there was. A new culture developed that was essentially Anglo-Saxon, and small kingdoms grew up.
By 600 about half of Britain was ruled by Anglo-Saxon kings. The kingdom of the East Saxons, or ‘Essex’, extended over the area that became the counties of Essex, Middlesex and much of Hertfordshire, including the site of Roman London.