So the OP has both sides of the argument, I would disagree with this assertion. The kings of Strathclyde - or Alt Clut ('Rock of Clyde'), as it was originally known - were a major force in early medieval politics. Strathclyde lasted much longer than any other northern British kingdom and in the tenth century, the kings of Strathclyde exercised authority across most of what, until 1974, was Cumberland. The border was at Stainmoor rather than Gretna.The flooded areas in Cheshire and Lancashire effectively cut off Cumbria and Strathclyde from the british west. There were places like the Eden Valley where land could be farmed but they were remote from either british or anglo saxon kings and so enforcing control over them, to extract tribute, was a lot of trouble for not much gain. It did happen, particularly under the Bernician Aethelfrith but he over extended himself and whilst, on paper, he pushed the map of his kingdom to the irish sea, he withdrew his men and his control was nominal. The land was of interest to people who wanted to carve out an existence for themselves, remote from kings and kingdoms for whom it became a sort of 'siberia'. They didn't really want it and were mainly concerned with making sure nobody else had it.
Flooding did not, in any event, impede movement or cut people off from one another. Seaways were major routeways, not barriers to movement.
In addition, early medieval politics was local and regional. Flooding or not, the kings of Wales had never exercised authority over the north. The socio-political situation in post-Roman Cumbria (by which I mean the English county of Cumbria) is shadowy, but there is just enough to argue that rather than being remote from British kings, the area was controlled by local British kings, at least until the seventh century, when those kings appear to have become clients or allies of Northumbria. It was held successfully too - the North West has little evidence for the Irish influence which is so visible elsewhere (especially South Wales).
There was movement between Northumbria and Ireland and between Northumbria and south west Scotland, and the easiest way to get from one to the other was via the passes that cross into the North West - notably the Tyne Gap, where Hadrian's Wall runs. The Tyne Gap is the main routeway from east to west and linked Northumbria to Carlisle. Carlisle's nunnery was headed by the King Ecgfrith's sister in law and the Queen of Nothumbria was there in 675, awaiting the outcome of her husbandl' diasatrous battle against the Picts. Maryport had been a Roman base and probably a naval installation and may well have been the point from which Christianity spread to Galloway (it is intervisible from Whithorn on a clear day and sailing across is far, far quicker than walking or even riding). The Eden valley is the main routeway north from London to Scotland - Scotch Corner on the A1 is so called because it is where you turn left for the quickest historical route into Scotland.
The earliest piece of post-Roman insular poetry seems to concern what is now Cumbria and it is quite possible that the earliest surviving Welsh panegyric concerns the same area. The two best examples of early Anglo-Saxon sculpture are at Bewcastle and Ruthwell.
This was no Siberia or forgotten backwater. For some centuries, Northumbria was the pre-eminent English kingdom, able to exercise authority over much of Britain. The North-West is Northumbria's direct neighbour and was connected to it physically, culturally and politically.