Wirelessly posted (BlackBerry9630/4.7.1.40 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/105)

BF upgrades are only debited if the TATL clears. But upgrading on LIS-LHR would be partner metal so probably you will pay for the upgrade no matter what. But that would also require a *A upgrade from a B or Y fare, not a discounted fare with cash+miles.

In other words, you are not going to get the LIS-LHR upgrade anyways so it is a moot point.