Have you seen if you could book the DFW-LHR at the same award price and then call in to add an earlier IAD-DFW segment to give you a longer layover in Dallas?

I'm a bit crazy, but if time isn't an issue then I'd actually prefer to backtrack to DFW to get more sleep. JFK-LHR is a six hour flight as you said, which gives you about 3-3.5 hours of sleep. DFW-LHR, on the other hand, allows you to have a full meal and still sleep for 6 hours.