Tomorrow I will be traveling BOS-LHR on AA and LHR-MIL on BA. At LHR how do I transit from T3 to T5? Do I need to exit passport control and reclear security or are the terminals connected for connections not requiring you to clear customs. I have a 2 hour connection (exactly 2hrs) so I hope it is easy. Anyone can help? I am assuming AA will tag my bags for the connection since BA is a one world member.