When we traveled often between DC and London, we were happy to use United's daytime flight on the way over. It cost a bit more than the overnight non-stops, but it was so much more "humane" not to lose a night of sleep. Today I recommended it to a friend for an upcoming trip, but when I checked the schedule just now, it's no longer listed. I can't imagine why that flight would be cancelled -- it was always seemed super popular, for good reason!
Called United -- they confirmed they don't offer an am nonstop from IAD to LHR this month or in December, but could not address what happened. Does anyone know?