Originally Posted by
KARFA
This is correct. BA will not approve the use of the GUF if your TA sends the booking to BA including non BA flights.
Whilst factually correct, a TA can send a booking to BA to approve a GUF before adding non-BA flights onto the booking afterwards. Therefore, it is possible to GUF a booking that does contain non-BA flights, just that you need a willing and capable TA.