The title BRETWALDA appears in the 'Anglo-Saxon Chronicle', against the year 827, and is usually translated along the lines of 'Ruler of Britain'. The 'Chronicle' itself provides the 8th holder, Ecgberht of Wessex (d.839), whilst quoting the previous seven from Bede's 'Historia Ecclesiastica Gentis Anglorum' (Ecclesiastical History of the English People): Bede, who didn't use the word Bretwalda himself, says that these kings:
"... ruled over all the southern provinces that are divided from the northern by the river Humber and the borders contiguous to it ..."
The 'Chronicle' substantiates this geographical distinction when describing Ecgberht, who, it asserts:
"... conquered the Mercian kingdom, and all that is south of the Humber, being the eighth king who was sovereign of all the British dominions."
"sovereign of all the British dominions" is James Ingram's translation of Bretwalda.

Obviously, those Bretwalda who were kings of Northumbria would, by default, also rule territory north of the Humber, but Bede notes that Edwin:
"... had the overlordship over all the nations who inhabit Britain, both English and British, except only the people of Kent ..."
Furthermore, Bede states that, at the time of his writing (731):
"... southern provinces, as far as the boundary formed by the river Humber, with their several kings, are subject to King Æthelbald."
Yet he does not include Æthelbald (of Mercia) in his list, nor Æthelbald's predecessor, Wulfhere (d.675), who might be supposed to have fitted the bill. After Bede's death, the 'Chronicle' doesn't consider Offa of Mercia (d.796) to be suitably qualified. Just what constitutes a 'Ruler of Britain' is, therefore, not easy to state categorically, but it should be considered as a poetic device rather than an accurate statement of fact.

Translations:
Bede 'Historia Ecclesiastica Gentis Anglorum' by J.A. Giles, revised by A.M. Sellar
'Anglo-Saxon Chronicle' by Rev. James Ingram/Dr. J.A. Giles