having it interlined only means that the lugguge has been tagged to your onward flight and the onward carrier is happy to accept it but in LHR there is an additional requirement from the UK Department for Transport regarding passenger baggage reconciliated which means that the lugguge can only be onboard if the passenger is on board and that means that the numbers have to be in the system for the baggage to be accepted.

In the USA, they do not operate passenger baggage reconciliation and if the bags are tagged onto a flight, they get loaded even if the numbers are not in the system and the passenger is missing.

How do i know this, I used to work in the Lost baggage department for AA at LHR T3 and everyday we used to receive bags before the passenger arrived.

eg if the pax checks in for a late evening departure from jfk to lhr very early, the bag would be loaded onto an earlier flight and arrive into LHR before the pax. they also use bingo sheets to load bags onto the aircraft, they dont have any records about which container and which hold of the aircraft the bags are located in whereas in london every airline uses a system called ultraBRS (Baggage Reconcialition System) which tracks the bag onto the aircraft and tells us which container and where the bags are located onboard the aircraft. This all happened because of failed El Al and lockerbie bombings which were both driven by explovises inside checked in baggage.

The way UltraBRS works is that the loader has to scan each bag tag and then will receive the go ahead or not from his handset. If the bag tag numbers are in the system and the same tag is scanned prior to boarding, it will be ok.

on the otherhand when the loader scans the bag tag but the number is not on the system, the tag will go 'red' and an alert will be issued and the bag will not be loaded onto the aircraft because they cannot reconcile it with a passenger onboard. This way the loader has to inform the staff at the gate and try and get the bag tags from the passenger and enter them into the system. If the tags are subsequently entered into the system, ultraBRS will stand down the alert and give the go ahead for the lugguge to be loaded. This is why some passengers are asked to show the bag tags just prior to baording in London.

if you say you have had separate bookings, im now guessing that when you boarded at the gate, the agent must have entered the numbers into the system, with or without your knowledge.

also this system allows certain bag tags to be blocked from boarding and i know AA has security agents who go through the list and pick out suspect names, bags tagged from third world countries etc and have the bag sent to a more detailed x ray machine and also wait until the passenger is boarded.

now if you are transitting london from the US, always make sure that the bag tags number is in the system. Most AAgents in the US, especially in the remote eagle stations and some mainline stations are 'thick as ....', they think they can interline with any carrier by just adding the onward flight onto the bag tag but it is not as simple as that especially in the UK. Just to add these same agents also send people to the UK without right documents (without Visas, assuming Green Card is good for the UK etc) to enter the UK, i have dealt with many passengers denied entry into the UK by the immigration authority and the aftermath. Generally the the AAgents and fellow kindred in the states are ignorant over the rest of the world and assume that 'if its good for america, its good for the rest'.

p.s. this is not an anti america rant, just want to highlight the differences in the airport operations and why the UK is strict because of bombings and plots in the past and also highlighting the general ignorance of the American airport workers who look bored and just waiting for retirement.

I again add that there is no way to transfer the bag tag numbers