Originally Posted by KARFA

A GUF can be applied at the time of booking or at a later date to an existing booking if the booking was made directly with BA.

For a TA booking the GUF is applied at the time of booking. Basically the TA notes the GUF on the fare quote and then sends it to BA to ticket - more information on the trade support page here https://www.britishairways.com/en-gb...d-card-upgrade