Under the current Bermuda II, no other carrier CAN take them on. MIA is not a designated US 2-airline gateway meaning that only AA can serve MIA-LHR on the US side for now. I don't think the MIA-LHR traffic figures have reached the appropriate threshold for the second US carrier to be allowed in yet.
Fortunatley, those regulations will likely not be around for long it seems, though you are absolutely correct. United/bmi will gladly enter this market once they can. MIA is now 6th largest US-London market, after New York City, Boston, Washington, Chicago, and Los Angeles, and just ahead of San Francisco, and has been fortunate in that it has not seen traffic fall or schedules reduced in the past year.