Apple tells everyone to use the app, and then can't get it up for everyone at the same time. I've heard people got on it at 8:01 and preordered, but I couldn't get on until about 8:15 by which time it was 4-5 weeks. Got a 2-3 week preorder in via the website but I didn't have anything saved there so it took time to log in etc. App had favourites setup, touch ID etc.
Have also preordered with a new contract with my operator which suprisingly isn't too expensive, but it will require some fiddling with moving my sim number across to PAYG temporarily.