It is in the T&Cs I am afraid and BA are clear that the discount applies only to BA flghts on BA metal and does not include codeshare.
Bizzarely this includes the JSA route to OZ where they share revenue and costs with Qantas.
I recently priced a flight to CGK using BA and got a fare 75% less when booked as a through fare and foregoing the discount.
Nor does it apply to BA iniquitous fuel surcharges or other fees which means if you are buying a flight LHR to GLA with reasonable notice the discount is negligble hardly worth the effort of getting to the log in page.
The discount could however be almost £700 if buying a point to point F class fare on BA.com.