Going BA domestic-AA international, T5 -> T3, all on BA ticket
LBA-LHR, LHR-LAX, 3 hr layover
I presume all boarding passes are issued at LBA by BA staff, and bags checked through.
AT LHR, do I need to stay airside and follow signs for Flight Connections?
Or just get the bus to T3 and either check-in with AA as normal, or go straight to security if I have a boarding pass for AA.