If you are still stuck , let me tell you how I did it pal. Scotland is not blessed with Great Cavalry so you have to try to build a fortress or a citadel to acquire Feudal Knights which is the strongest unit of Cavalry you will get. Also you need to start attacking Norway and curb their expansions early in the game at any costs. Don't stop till they are finished and keep using the cavalry units to destroy their strong infantry and huscarls units rapidly. Then try to forge an Alliance with the Welsh and move downwards to Britain. If the Baron's Alliance gives you a mission take it but don't deliver the city to them. Keep it for yourself. They are not worthy in the end.
Keep building roads, Ports and Markets in all cities to keep the revenue going. Never get too carried away by building just units as it's really easy to get bankrupt playing Scotland in this Campaign. Now as you strike down on England William Wallace should be appearing . Keep him alive at all costs as he is your lead commander in conquering England step by step. Eventually after you conquer England , the Welsh are likely to betray and declare war on you. That shouldn't be hard since you would already have most of the Island and can beat them with Cavalary units especially Feudal ones. Once you get a grab on the Island. Dublin should be your first target in moving on the Irish coast. Take it over and start pouring in units from Scotland and Wales into Ireland till you get it done. Good luck dude
