After seeing this Idea I've come up with a little solution to many peoples problems.
Buying a phone currently requires you use a credit/debit card, where the shipping and billing address are the same, and it must not be a business address (even though some businesses have flats above them too).
What would be good is if you can use credit to purchase a phone.
So you can top-up like normal, say, £100, and then buy a phone.
Or, take your Payback as Credit, then spend it on a phone.