CONFESSION TIME: This is the only scene, EVER, where I actually disliked the 10th Doctor. Don't judge, I was a silly girl back then. A VERY silly girl. Now I look back on this scene with love and happiness. It's always hard when a Doctor regenerates.
This makes so much sense...maybe she's his wife, his real wife, and that's why he loves London so much, because that's the last time anyone ever remembered seeing her before the Time War, she was in London with the kids.