aye, i have sought out one of the most difficult challenges known to AH. IF you choose to accept, you are to come up with a plausible scenario in which the nation of Strathclyde survives with a POD no earlier than 450 AD.
A good starting point might be to confine the Scots to their west coast colonies and allow the Picts to exist as a separate Kingdom. Scotland would then be split into 4 parts, Strathcyde, Scotia, Pictland and the Norse dominated Western Isles. They would have varying relations with the English to the south who despite the Viking wars of the C9 -C10 you should keep split into 3 or 4 Anglo Saxon kingdoms.