A BA paid upgrade in advance of flight is a POUG and you will probably not be offered one on a TA booking. See:
https://www.flyertalk.com/forum/brit...24-thread.html
BA does sometimes offer airport Upgrades at the airport on the day of flight. See:
https://www.flyertalk.com/forum/brit...24-thread.html
If you are trying to upfare your ticket you should probably follow the advice above and contact your TA.