Reference:
I don;t think the only way to get in UBB from BB11 was to qualify as the winner...
Davina McCall repeatedly kept telling us "the winner" will go on to UBB (or at least have the option to.) So if the BB11 winner had declined to return for UBB the house numbers would be the same as they are now. But I remember Davina regularly telling us the BB11 "winner" would go on to UBB.