Originally Posted by
corporate-wage-slave
What it boils down to is that deploying UUAs and GUFs mucks up the ticket structure
Years ago they began to allow MFUs (as they were) on BA sectors in bookings with other carriers included. Since then, the GUF restriction has never made any sense to me. Similar to the restriction that you can't use a GUF via a TA on a fully paid ticket which has nothing to do with Avios except earning them, if you're in a HHA.