BA does indeed have a policy that TA issued (on BA stock) booking cant be applied to booking with other carriers.

In practice, I dont think it is enforced. I contacted propeller and they were able to upgrade
BKK - HKG (stop) - xLHR - AMS - xLHR - HKG (stop) BKK from PE to J on the 4 BA operated legs. BKK HKG BKK is on CX E class and left untouched.

Also, I dont believe it was issued in the country of my parents BAEC membership as well.