IAG will be broken up. Not sure that it'll have anything to do with Brexit but each country will want to save their national carrier. I imagine that the British government will foresee a need to have a national carrier post "transition ".
I mean, how can they plan on trade with the far reaches of the commonwealth and the world without a practical Airline.
I think that we are rapidly going beyond the point of believing that IAG can survive in it's own right. Maybe some are still clinging to hope.