1) Join iUP as you would - yes
2) Make the 11 payments as you would - yes
3) When a new iPhone is released, order for delivery and pay the full amount upfront - No, you can go via the App Store app, select the device you want then chose a time and location to upgrade - you won't be charged but if you fail to tuen up by close of business then you'll lose the stock allocation
4) In store, cancel the existing iUP by returning the phone. This cancels the iUP, basically - No, you can only cancel the existing iUP by upgrading in-store. You'd need to call Barclay's to pay off the balance which will cancel the iUP.
5) Also in store, do the return-refund-join iUP with that iPhone- dance - you can also do this, no problem, they will even do the iUP before the refund incase it rejects as once they refund the device it cannot be sold again.
6) Repeat