Originally Posted by
DoB840
I'm actually asking about JFK and FCO, specifically - I'm connecting at LHR in both directions, not originating there.
it doesn’t matter if you are originating or connecting. You still can’t get in to the airside departures area unless you have a bp for a flight that day. Departing and arriving passengers are segregated. If your onward flight is the next day you will have to exit through arrivals and go landside.