Your challenge if you wish to accept it is to create a plausible scenario in which the North Britons to survive as a strong polity beyond the early medieval period (1000AD/1100AD)
No POD is allowed before the Battle of Chester 615/16 AD...
Furthermore, while the Kingdom of Strathclyde did survive beyond the year 1000 AD, it did not survive fully into the High Middle Ages nor leave much cultural remnants behind.