Scotland would be interesting as a barbarian kingdom. It could be a dangerous military adversary of England, when the King manages to unite all the clans, and at the same time a lawless adventuring ground. There is a long tradition of such descriptions - eg Walter Scott.
You have outright primitive Celtic barbarians - Highland Clans, half-civilized treacherous bandits - Border Clans, and in the middle most civilised Lowland Scots - the source of the power of the king - but the rural areas are still ruled by mostly lawless lords and knights. In cities there is a certain tendency to heresy, religious enthusiasm and witch-burning. Highland Clans are nearly Pagan.
Pics should be based on Machen and Robert Howard (Worms of the Earth). They used to be a barbarian tribe worshipping Chaos gods (as in Howard's Conan stories), but the sorcery and Chaos worship caused them to degenerate. They are now soulless creatures of Chaos; when they emerge on the surface, they assume a mostly human, if degenarated shape, but underground and even above ground at night they tend to change, sometimes subtly, sometimes quite obviously. They can became mostly animal-like. When the influence of Chaos grows, esp during ceremonies, sacrifices, their hands and legs can fuse together and they can become snake-like. During the greatest ceremonies with human sacrifices, whole tribes can fuse together into one ooze-like creature.
Picts like to abduct women and children, for use in sacrifices, but primarily to keep their genetic stock from degenerating so far they become permanently an ooze. Their chiefs are most human of them - they marry exclusively abducted women.
Picts can be threatened into cooperation, but cannot be converted from Chaos-worship. Any attempt is meaningless, since they think exclusively in Chaos-categories, and cannot even understand what it means not to worship Chaos. This applies mostly to chiefs and sorcerers, because other Picts speak only in their own language, which to all non-Picts seems to consists only of animal-like screeches and is unintelligible.
Consequently, Picts have very primitive technology - and cannot be taught anything higher - but powerful magic.
Kings of Scotland have some kind of pact with them and occasionally use them in war, but try not to do this too often or to be too obvious about it - Chaos worship is not that popular amongst their (other) subject, esp. the witch-burners of cities.