Having just returned from London last night, a helpful POI for those leaving for London (LHR) soon. Sometime in late March (i.e. around now), American is dividing itself between Terminals 3 and 5 at Heathrow. Flights to/from NYC, DFW, and Miami will be through T5 while other flights (i.e. Philly, etc.) will be in T3. Triple check your arrival and departure terminals if you are scheduling a pick up or pre-purchasing a Heathrow Express ticket.