Slightly off topic - on IB/BA transfer
I know IB account with UK address can transfer freely between IB and BA.
However IB account with US address does not seem to have such privilege.
(I looked at that probability when e-Reward has a much higher redemption ratio on IB and any other options worthwhile to me. Then I found out it has no use because I cannot transfer the IB Avios to BA Avios with a US address account.)
How can you be sure the BMI account with US address can transfer the Avios to BA?