What I would like to explore today is the hypothesis that the early Bernicians were viewed as outsiders by the southern English, including in Deira and Lindsey.
The Bernician heartland outside of the former Roman province may have also made them seem more barbarian to even the Anglo-Saxons. Was there a long held suspicion that these English were more of an Anglo-British mixture than the southern English? They may have been more distinctive. The northern British would have been less Romanized, so even if the southern English did intermarry more than we have evidence for, the upper-level Britons of the south would have been far more Romanized and may have considered themselves Romans, more than Britons. Recall that Patrick addressed his British audience as fellow Romans. As in Gaul, it may have been Roman ancestry that conferred status rather than native ancestry. After 400 years of Roman Britain, this would have only been natural in the most Romanized areas. Bernician adoption of British hillforts, like Bamburgh and Dunbar, would may have reinforced the idea that they were not as civilized.
While the Bernicians appear to have taken more British culture like use of the hillforts (and perhaps cavalry), and we know there was some intermarriage, it may have been their relationships with the Picts and Scots that really made them stand out. In the early 5th century, Geramanus of Auxerre was involved in repelling an attack by “Picts and Saxons”, which could be a coordinated attack by Bernicians and Picts. This would make more sense that a coordinated attack by Picts and continental Saxons. Britons called all the English (Angles, Saxon, Jutes, etc) Saxons. The Picts would have been natural allies of the Bernicians against local Britons as long as there were Britons between them. Their British neighbors would have been their continual enemies simply because they were continually taking land from them.
Peaceful ties between the Bernicians, Scots and Picts prior to the early seventh century can be seen in two sets of Bernician exiles taking refuge among the Scots and Picts in c. 600 and again in the 620s. Its likely that these were not the first exiles or hostages exchanged between these peoples. Indeed in times when Scottish or Pictish kings were extremely powerful, Bernician nobles may have been hostages in their courts. This would have fostered intermarriage and ties between Scottish, Pictish, and Bernician kings. Pre-Christian kings were polygamous and bride exchanges would have been common. The Bernicians may not have been powerful enough to resist giving hostages to more powerful neighbors before Æthelfrith.
Not until Ecgfrith with his half-Kentish mother and his East Anglian bride (and then perhaps another southern bride) did the Northumbrians become really integrated as part of the English. His reign is the first to embrace Canterbury throughout. As far as that goes, Ecgfrith is the first that we know to enforce his northern hegemony by war. It is possible that Oswald and Oswiu fought battles to bring them into line, but it wasn’t recorded. We know that Æthelfrith fought against the Scots, but this was a rebellion of a minor king against the major (Scottish) king. Oswald and Oswiu appear to have gained and maintained their northern hegemony more by political manouverings than by war. This is not to say that they didn’t fight key battles, but they maximized their hegemony and its maintence by politics. From Ecgfrith on, northern hegemonies were gained and maintained only by force. Ecgfrith brought the Bernicians/Northumbrians into the mainstream of English culture and politics but at a high price.