This is all part of the ongoing conversations related to the new LHR's 3rd runway. It's all fairly common knowledge amongst the airlines and is discussed fairly widely. As you know, HAL had an (eye-watering) expensive project for another terminal and so on. However, HAL is being heavily challenged by the airlines and it's slowly warming up to the idea of not having a new terminal after all, but, instead, to just have the "toast racks" from T1 up to T5. T5 would be enlarged, and there are projects thinking at how this could happen. This could open the house to other oneworld airlines, especially now that IB is using its check-in system.

I'm not close to it, but undoubtedly there are a lot of challenges, and the timescales will be long (think at glaciers, tectonic plaques and so on). Besides, in the words of Boromir, "one does not simply build an extension to T5".

All this, Boromir quote included, is my own opinion and not BA's, or HAL's.