They hate each other
The 'Viking' faction is named as Norway, who held onto islands in Northern Scotland (Shetland and Orkney IIRC) until well after Wallace's time. Presumably that's where we'll see them in this campaign, though I don't know what time-frame it's going to be set in.
The new Britannia campaign does looks decent though. I would prefer if it had more factions vying for power of the islands.