The movie has been pushing the idea that being educated will get you ahead in life, but in the finale, Arthur pulls a sword out of a stone which has nothing to do with education. However, Merlin can see the future and probably knew he was destined to become King Arthur. And all that education will make Arthur a better king: Arthur knows what it feels like to be small and weak and surrounded by stronger predators far worse than even what you'd find among humans, and he has been taught to favor "brains over brawn", which will, Merlin hopes, allow him to pull England out of the Dark Ages.
Given that the film inadvertedly states that the year is 585, that would mean that the London where Excalibur is seen in is not Greater London (or Westminster), but rather the Roman City of London.
Congratulations, Arthur! You're a child king in Medieval England! You're basically surrounded by hostile nations, the country's been in a leaderless quagmire of indecision for most of your lifetime, your trusted advisor flies into a rage and ditches you at the very idea of you being involved in combat, and your only preparation for all this has been the dubious lessons of being turned into amusing animals. ...Good luck!note Thankfully, you're a Disney monarch, so you basically just get a vague, implied Happily Ever After. At least until they decide to make Sword In The Stone 2. Though on the bright side you might hook up with a human version of that girl squirrel!
As Frank Thomas and Ollie Johnston point out in their book The Disney Villain, Kay is what Wart would have become if Merlin hadn't entered his life, Wart hadn't pulled out the sword and stayed under Sir Ector's thumb—he would have eventually degenerated into a surly, cynical bully with a dull witted ego.
Madam Mim's status as The Dreaded, coupled by the way she talks to Arthur while trying to kill him implies that she's killed people before. And her reason for wanting to kill Arthur is extremely petty, so who knows how many people she's murdered for little to no reason in the past...
People who have seen how Arthur's story goes down will be having this by the end of the movie: Arthur's wife will cheat on him, he will dedicate his life to sending knights look for a mythical treasure but they will all die before finding it while the country slowly falls apart until he dies himself in a battle. Weee.