So here's a beginning for atimeline that i think sounds likely, but I'd like your opinion
579: Urien of Rheged, Rhydderch of Ystrad Clud, and Gwallog of Elmet are invited by Morgant of Bryneich to form a coalition to push back the Angles that have been gobbling up his kingdom.
580‐1: the campaign is proving successful, more successful than any of them expected, the angles have been driven from Bamburgh to their last fortess on ynys medcaut and much of it is due to the prowess and tactical mind of Urien who has gained much prestige. Worried and envious, Morgant pays one of his men to assassinate Urien.
Now for the POD: the man Morcant paid decides to do a bit of double dipping (unscrupulous men will always be unscrupulous) and rats out his employer for some extra gold. Urien waits until the island falls to his forces before exposing Morgant's treachery. Morgant's men by this point are sick of his poor leadership and depose him, swearing to Urien who is clearly the better king.
The rest of Ida's kin flee to Deira as the Welsh conqueror their fledgling kingdom, and Urien is more or less the undisputed overlord of the north, solidified with arranging a marriage between his daughter Morfydd and Rhydderch.
585: Urien dies on campaign in Diera, and his son Owein succedes him, though he has to deal with the king of gododdin trying to invade him while he's weak.
588: Owein, with help from his brother in law, is able to turn the tables and Goddodin is split up between Ystrad Clud and Rheged. Owein takes the king of Goddodin's daughter, Teneu, for a wife who gives him a son kentigern (who OTL is more commonly known as St.Mungo)
593: Owein resumes his father's campaign against Diera, while his in law raids the scoti of Dal Riata