From "Debrett's Kings and Queens of Britain" by Williamson, ISBN 0-86350-101-X, p 14: Later in the 5th century, other settlers established kingdoms throughout England, The kingdoms so established are known as the heptarchy (i.e. the seven kingdoms, although the number varied). Although the kingdoms were quite independent of each other, they formed a loosely knit confederation under the leadership of one king, usually the most powerful among them, who was elected as a sort of head king with the title of Bretwalda. The seven Bretwaldas listed by Bede were:
Aella, King of the South Saxons (Sussex)
Caewlin, King of the West Saxons (Wessex)
Ethelbert I, King of Kent
Redwald, King of the East Angles
Edwin, King of Northumbria
Oswald, King of Northumbria
Oswy, King of Northumbria.

From "Debrett's Kings and Queens of Britain" by David Williamson, ISBN 0-86350-101-X, p. 20:
The 3d King of Wessex, Ceawlin (reigned 590-593), is listed by Bede as the 2d Bretwalda or acknowledged overlord of the southern English.

Yorke's "Kings and Kingdoms of early Anglo-Saxon England". Weis' "Ancestral Roots. . ." (1:4). Leo van de Pas contributed to GEN-MEDIEVAL on 29 Dec 1998:
"Cealwin (King of Wessex 560-590). In 560 he became King of Wessex and, in the same year, Ethelbert became King of Kent. During Cealwin's reign, Pope Gregory sent missionaries to England and, in 591, Columba went to the Picts in Scotland. In 593 there was a great slaughter of Britons at Wanborough and after having been driven from his kingdom, Caelwin died."