Kilted raises a good point. The US rarely make a deal that provides a level playing field, let alone one where they might be disadvantaged. Yes it could be an advantage for bmibaby, but you can bet that the US carriers will receive a greater advantage.
Another thought, where will they get all the slots from if LHR isn't upgraded? Maybe, they can go into Stansted if that is the lucky on to be upgraded. Thus, the US carriers will get in via the "back door"? Alternatively, allow the US all the access to LHR they want while upgrading STN, making it the UK's primary airport. While they're at it, they can charge the US carriers an arm and leg.
Food for though.