Originally Posted by Cymro

BA always sets POS to the departure point. You could therefore book LHR-DFW-BNA with an I-class itinerary over the phone with BA; of course, depending on date, you might find that available itinerary ex-DUB is significantly cheaper with a UK POS anyway. (Or if you wanted to avoid the risk of a misconnect or having to collect bags, while still dealing with BA directly, consider flying from Belfast).