PE does exist on international DL flights as Premium Select, but it depends on the route you're flying. In your case, I'm assuming the DL metal flying from ATL to LHR are the A330-300s which is one of few configurations without Premium Select. If it matters enough to you, if they change you to DL, maybe you could ask to be re-routed to connect through DTW or BOS. You wouldn't get Premium Select from ATL to those airports but for the transatlantic portion those birds have Premium Select. But that seems like more trouble than it's worth.

Above all, shouldn't you be entitled to a partial refund for the difference in fare class if they end up rebooking you to C+?