Bernicia

Can any learned correspondent favour me with the name or title of any English nobleman who held authority in Wales, or the Borders, in 1370-80? The motive for this query is, that a poem of the time, by Trahaearn, a celebrated bard, contains the following passage: "Though fierce in his valour like Lleon, with a violent irresistible assault, he vaulted into battle, to plunder the King of *Bernicia*; yet the ravager of thrice seven dominions was a placid and liberal-handed chief, when he entertained the bards at his magnificent table." It is not supposed that the king here mentioned was any thing…