The recent thread on BA's computers current inability to retrieve info from partner computers made me think that maybe this info concerning BA to Iberia might be of interest to someone someday.
Recently we flew SFO to Granada Spain, with BA to LHR and Iberia from LHR to Madrid and then onto Granada as BA codeshares. When we checked in 24 hours ahead, we were unable to check in for the Iberia flights. BA wasn't able to check us in at SFO either-apparently, even tho BA owns Iberia, their computers are incompatible. We were told to get our Iberia boarding passes from the BA transfer desk at LHR, which they implied was in the main airport. However, the transfer desks are in the transit area, so you needn't worry about all the immigration hassles, altho you do have to transfer from terminal 5 to terminal 3.
The line at the BA desk was so long that we would have missed our flight if we had waited on it like we were told at SFO. Instead, we went to the much shorter Iberia line, and they checked us in and gave us our passes.
So, if you're on a BA codeshare with Iberia, and can't check in ahead of time, that is apparently normal, Don't go to the BA desk at LHR, go to the Iberia desk and get your passes there.