"Later the Orkneys (and other places) were invaded by the Vikings; also a group of Irish (the Scotti) moved on in to western Scotland, becoming the Highlanders and Islanders."
I had understood the Celts (Scotti) came much later than 5,000 years ago.
And I think the Nordics came later, too.
So that goes back to the picts or pre-picts; eg. what one might label the indiginous people of the British Isles.
(Before Celts, Romans, Agles, Saxons, Frisians, Jutes, Normans, Belgians, Vikings, etc.---5,000 years ago)
Actually, it's generally agreed that the Picts were probably a Celtic people. The remains of their stone circles and the inscriptions on those stones are near cousins to similar remains on the continent in known Celtic sites. Of course, the Celts may have simply conquered and absorbed the peoples who built the stone structures.
The Scotti migrated from Ireland in the 6 - 7th centuries, and the Vikings began their invasions in the 7 - 8th centuries.
The original Celtic invasions of what became known as Britain and Scotland would probably have been 1,000 - 1,500 years earlier. It is known that Celtic tribal migrations continued up into Roman times, as various tribes tried to escape Rome.
It is highly likely that there were earlier invasions of other peoples that are lost in the mists of time, just like the rest of Western Europe.
Quite correct on both accounts. Those were the big waves