Thai is correct. United does not fly any DC-10's at all across the Atlantic. Flight 920 from IAD to LHR is a 777, it used to be a 747-100 (I remember that very well because I've been on that flight four years ago) until they got their first 777. Westbound flight UA921 was the very first route flown on a 777. You should know that, across the Atlantic, United flies only 767's and 777's.
Have a good time if you are going to fly the friendly skies soon.