I used an old Orange number that I topped up in November. However, only now has their online system decided to approve for the upgrade price. This means you don't have to worry about buying any extra credit and it is unlocked because its from Carphone Warehouse.
Also, blue is finally in stock because I have got my dispatch email in the past couple of hours.
So if you were declined on an upgrade last year, it's well worth giving it another go.
£160 + £10 topup otherwise.http://www.carphonewarehouse.com/phones/nokia-lumia-735/ppay
Blus is OOS