Originally Posted by corporate-wage-slave

You go to the check-in of the flight operator at any stage in the journey (sometimes that is the same as the marketing airline, but that would just be a coincidence). So if IB aircraft are flying the first leg, irrespective of flight number, you go to IB. The baggage tags ought to have the same flight numbers as the operating airline, but actually it shouldn't really matter since baggage handlers, and certainly LHR baggage system, should be able to interpret codeshares correctly, after all there won't be any other jet for which the bag(s) could be loaded.