Here's the skinny. INTL you must have a seat in the back on a pax carrier. You still "jumpseat" but you basically become a non rev. As mentioned UPS/FDX don't carry intl OAL jumpers. Polar/atlas/Kalitta and maybe Southern(???) might. But you have to contact them directly for a schedule. Past experience on polar has been excellent, never tried the others. You may need to go to STN or even EMA to get on.

You should check with your jumpseat coordinator and get a def answer on who you have an agreement to go intl on. DL out of LHR is pretty easy (space permitting), just have to go to the right counter. UAL intl is also been no prob in the past, but never taken them out of LHR. Never tried US and never had an AA intl agreement.