Dál Riata
The ruler of Dál Riata was King Áed mac Echdach (Áed Find) (probably, the start of his reign is shrouded in mystery).
Dumnonia
The ruler of Dumnonia was King Oswallt ap Cawrdolli (probably, his reign's exact dates are unknown).
Kent
The ruler of Kent was King Ecgberht II, who had ruled since 765 and would continue to do so until 779.
Mercia
The ruler of Mercia was King Offa, who had ruled since 757 and would continue to do so until 796.
Northumbria
The ruler of Northumbria was King Æthelræd, who had ruled since 774 and would continue to do so until 779.
Fortriu
The ruler of Fortriu was King Alpin II, who had ruled since 775 and would continue to do so until 778.
Wessex
The ruler of Wessex was King Cynewulf, who had ruled since 757 and would continue to do so until 786.