pax britanica
Wrong in the case of Aeroflot actually there are two more important reasons lessors want access to their assets quickly if needs be and Russian import duties are astronomical.
Corporation tax is the least of their worries as Putin and mates sort that bit out !
IAG was floated in both Madrid and London reprentative of the valuation of BA and IB at the time of merger.
BA retain full day to day control of their and UK subsidiary operations from London record profit and loss and pay relevant UK taxes. Iberia follow the same principle is Spain.
Also ensures retention of National Traffic rights beyond the EU Morocco Israel Switzerland and US that might be break bilateral treaty rules with a fully merged business
You can not buy shares directly in either group company today as these are all held by IAG from original valuation and split
IAG is the holding company set targets authorise major expenditure and is the publically listed company
One thing this set up does allow for is relatively speaking an easy process of demerger or even closer of either BA or IB or indeed EI or VY if deemed in the interests of IAG at any point.
All that said I cannot for the life work out what Niki brings to the table at the moment.