You will always have to pay the UB airport tax ( Passenger Service? charge ) of about USD 30 out of LHR in each direction. The APD charges are a bit more complex in that they do not apply if you are on a confirmed ticket showing both connecting flights on the same ticket however standby tickets are normally treated as two separate non-connecting flights ( as you may not get accepted on the connecting flight ). Its normally up to the ticket issuing carrier to make this call as it is that carrier who will be charged the APD if the authorities deem it is applicable.