England's starting position in the midst of losing the 100 Years' War is not easy for the AI to handle, and they can easily suffer a period of early weakness where Scotland is able to win a war and take some land/release Cornwall.
That said, even after this point, England is still bigger than Scotland, and by far the biggest power on the British Isles. I have seen AI England lose that early war to Scotland and then proceed to turn around and destroy Scotland in a retaliatory war 20 years later once they have rebuilt, because they still have more manpower and income. This has happened a couple times in games that I have played, and then England goes on over the course of the game to annex Scotland and form GB. I've also seen England be relatively strong from the beginning and take out Scotland early, and I've actually only seen AI Scotland completely dominate England and form GB one time.
Obviously, this is all anecdotal, but I don't see England getting destroyed very often, and even if they are weakened and lose the first war against Scotland, they generally seem to still be more powerful than Scotland afterwards, and can make a comeback once they've gotten past the messy early years.