It's actually even worse if you're trying to ENTER the upgrade programme. You can't order a new phone via any other means, leaving no choice but to wait for stock to arrive in store.
The only way to do this is by continuously checking the reservation page, which in my opinion is very badly designed as you have to check each store individually. For London, for example, there are 5-6 stores within reasonable distance of most residents. There are many more only just outside London.
The way this *should* have worked:
1. Using app or website, select your desired model/colour/size
2. At the payment options stage, select upgrade programme
3. Pay down payment if relevant
4. Choose store pickup or delivery. If store pickup, choose store(s) local to you.
Even if the app purchase process couldn't have been streamlined, the reservation page could at least let you choose the model first, then list the stores having stock (the reverse of the current design).