Getting dressed up like a dogs dinner for the ballet, or for the theatre is a dead giveaway that she's not a proper toff. Most properly "posh" people go to the ballet and theatre regularly and usually are fairly dressed down. They might dress up a little, especially if it's an occasion, or special show but in general less is more. Often people have to get the train/tube home, so generally wear a good coat and boots. Her looking totally out of place absolutely gives away her lack of class and the fact she's a novice at attending the ballet.