The flights operate with IB codes in the MAD - LHR direction.
Flights between LHR and MAD are operated by Iberia aircraft using Iberia callsigns in both directions.
They all have BA codeshare flight numbers too, of course, but the difference is that in the LHR-MAD direction those are in the normal range for BA-operated flights (e.g. IB3171 masquerades as BA520) whereas in the MAD-LHR direction they are in BA's codeshare range (e.g. IB3162 pretends it's BA7054).
I have no idea why the difference.