All DL service in/out of LHR is on fully flat seats, regardless of destination in the US. They run mostly 767s on those runs. The 330s that you may find are ex-NW eqpt, and still have the angled flat seats, although they are allegedly sched for refit by Summer 2014. Also, If you are traveling BizElite on trans-oceanic, there are ticketing classes where your domestic US connections will be automatically upgraded to domestic First. Also, an ATL arrival from LHR will likely come into E or F in ATL, and a domestic to DFW likely departing from A or B. A bit of a hike, but an efficient airport that handles it’s volume (and connections) far more gracefully than LHR.