Since Iberian peninsular BA routes (& OneWorld) are projected as 757 ex-T3 once T5 opens, it seems likely that we will see mainline take LHR-AGP etc. whilst a blend of Airbii do it from LGW.
Club loads and load factor might even see a 767 on part of the LHR schedule...
Dangerous to allow logic to creep into the plan though!