is traditional and well-known, but there was never in fact any time when Anglo-Saxon
England was divided among these seven kingdom
The dates of foundation and early history are often poorly attested; Bede and the Anglo-Saxon Chronicle help fix dates but do not do so with certainty, especially for times hundreds of years before they were written, which is when the earliest Saxon kingdoms came into being.
Often one kingdom was a vassal to another or was temporarily occupied by it. There was a title of Bretwalda held from time to time by one of the kings as a sort of honorary high king of England. The position became fixed in the dynasty of Egbert of Wessex, whose successors became the kings of England.
A number of smaller kingdoms not part of the traditional hierarchy existed for part of the time. The kingdom of the Hwicce around modern Herefordshire existed from 628 to 788 as a dependent of Mercia, and the kingdom of the West Angles or Magonsaetan existed above it on the Welsh border from about 645 to 725.
There was a kingdom of Lindsey, modern Lincolnshire, founded about 550. This was absorbed by Mercia about 790. Mercia gained ascendancy over Lindsey in about 715, Essex about 730 (when Mercia took its capital London), East Anglia about 740, Sussex about 774, and Kent about 786.
Mercia and Wessex were the only kingdoms to survive the Danish occupation, and by Mercia's acknowledgement of the overlordship of Wessex in 829, the reign of Egbert of Wessex as first King of England is considered as beginning.