I think Iberia might only operate from Barcelona and Madrid to LHR (check iberia.com) but it did transfer other flights to Clickair (now Vueling) some time ago.
I'd have thought there's no room in T5 for Iberia flights. There's never been any mention that these would move to T5 if the merger went ahead.
A problem for T5 is that it was never going to have the capacity to handle all oneworld flights.