With Scotland the key is to take out England as quickly as possible, as a lot of posters have said. Taking either Nottingham or London will cripple the English and improve your own position enormously. Scotland are at a disadvantage against the English but most factions are at the start, this is why the early game is so fun.

Once the British Isles are secure, the Scottish are in a very strong strategic position. I found I could expand very rapidly but that was because I had modded the pikemen by removing the secondary weapon, which made the Scottish units very formidable indeed. This only helps in the mid to late game however, as you can't build any pikemen until you have a fortress or a city.