Why don't you ask if you can be rebooked on AA-IB-AY joint- venture flights? It might work and you might be able to avoid LHR. BA are sometimes reluctant to do this, especially their offshore call center staff, but my experience is that AA is always willing to re- route on JV flights ( their US based staff are so much more helpful). Don't just suggest a reroute, try and find the flights you want and ask to be rebooked on those.