:/ Damn that sucks. Change of address might be the issue, especially if it isn't reflected on your documentation. If Barclays can't easily verify the info you give, they'll probably be more inclined to reject than take the risk. Might be worth contacting them to see if they can offer any insight, Apple won't know why it happened.
Also as someone on the iPhone Upgrade Programme, your story has alarmed me somewhat

. I was accepted without issue last November for my 7+ 256GB, and have switched jobs since then for more money, but I'm still anxious about going in to upgrade.