Hi. Let me preface this by saying I work for O2. I just wanted to elaborate on the registration part in your story.
If you're an O2 customer connected to the mobile network when you first open the app, you won't have to enter your mobile number or go through the activation code step. We'll detect your mobile number automatically and save you the hassle - and we obviously know you're with O2.
We can't detect your mobile number if you're connected over wifi, or if you're on another mobile network. Hence you need to type your number in, and we will validate it by sending the activation code as a security measure.
Hope that helps.