Originally Posted by
gypsyjaney
There really is no excuse in this day and age. The details are already on the BA system for goodness sake.
If they're in your BAEC account properly, then MMB should auto-populate. It works for me, and it seems to work for most of us who expect it to do this. So you might want to check that part of your BAEC account.
On the routes where you have to re-enter the API after confirming your seat but before printing a boarding pass, you just have to do it.