she still needed to go to England becaue she took a human life and she needed to learn how to control all her dark power.
I think that if Giles would have stopped her in the magic box, it would've cheapened the rest of the characters. Xander did not get to save the world, Anya would not have shown so much compassion for Giles, Buffy would not have asked Dawn to help her. And I always felt that if he could have, Giles would have killed Willow. I think if he would have stopped her, he quite possibly may have killed her. But who knows?
I like the way it ended. But good point to wonder what would've happened