Was it clearly sold as with free changes on the day? I have seen a couple of "Plus" fares on LHR-MAD (most often when looking the other way around, MAD-LHR-MAD tickets) that include a bag (you can't get HBO on that route) but do not offer free changes on the day.
If so, then IB shouldn't be a problem, as I have swapped from IB operated flights to BA operated flights a couple of times, in both directions: LHR-MAD on BA "prime" operated by IB but really with a BA flight number (that weird arrangement...) and proper codeshares on MAD-LHR (BA7xxx to BA461).