'Highland' and 'Lowland' are cultural terms dating to the 14th C or so and pretty much synonymous with Saxondom and Gaeldom. The geographical faultline has never perfectly matched this divide (parishes in Caithness were considered 'Lowland' in the 17th and 18th centuries, for example) and certainly didn't then, when much of the southwestern backwoods were Gaelic-speaking, never mind in early medieval times when Welsh is into the mix as well.
The great divisions in Scotland - which was by virtue of geography always less centralised than England - in about the early 1000s were Alba between the Mounths, the Forth, the sea, and Drumalban; Moray north of the Mounths and taking in much of the northern Highlands, whose rulers were the great rivals of the Scottish kings; Welsh-speaking Strathclyde, where the heirs to the throne appear to have ruled by some system of lateral succession at some point; recently conquered Lothian; a Norse-Gael kingdom in Galloway which felt able to navigate between Scottish and English allegiances; and the Norse rulers on Orkney and the Hebrides, who were involved in the power-politics of the region but were actually subject to Norway. Argyll and the western Highlands seem to have been dominated by shifting clannish powers aligned to one dynasty of the other.
Hey, put it like that, and it begins to sound like there
was such a period, no?

If you want to stretch it, the whole northern Norse-influences world was full of intimately-connected power-centres all struggling to get the top-spot, which Sveyn and Knut briefly did, up until 1066. And that northern world, though I can't vouch any exact figure, probably had fewer people in it than the empire of Charlemagne.
In early Anglo Saxion times there were if I remember 10-14 little kingdoms,which had by the time of the viking raids had combined into 5-7 kingdoms.
You would need two things to have happened
1. King Swen Haroldson and King Canute Swenson of Denmark would not have conquered England
2. No Norman invasion, Maybe have William the bastard drowned at birth
But their exploits were possible because the Saxons had already established a pretty organised state for the time. They could beat the army, ride into London, plonk down on the throne, and then crush or subvert a sufficiency of rebellious magnates and bam, they were kings of England.
The Great Heathen Army couldn't do that, precisely because the kingdom of England didn't yet exist.