The ruler of Alba at the beginning of 877 was King Constantine I (Constantine the Wine-Bountiful), who had ruled since 862. This year, King Áed would take over.
The ruler of Mercia was King Ceolwulf II, who had ruled since 874 and would continue to do so until 883.
The ruler of Strathclyde at this time was likely to have been King Dyfnwal.
The ruler of Wessex was King Ælfræd (Alfred the Great), who had ruled since 871 and would continue to do so until 899.