Strongly suspecting wishful thinking by a competitor here.

Moving BA out would mean moving AA, CX, AY, MH, RJ, IT and QF (assuming JL and IB are moving to T5 anyway). BAA would be being very courageous (to borrow from Yes Prime minister) to do this. Unless, Heathrow East space is available instead. T4 is a non starter for BA, the runway crossing plays havoc with the schedule.

Adding a further mentioned point, the JSA may well be going, but it's only 6 daily flights. The remaining oneworld members want BA cohabiting terminals too. Moving to Heathrow East wouldnt worry them much but moving BA out of T3 without the rest of oneworld is unthinkable.