What time is your flight? UA has a ORD-IAD-LHR that is all the same flight number. I can't remember what the exact times are but I am pretty sure this is it. I few it a while back (on the same deal) and the purser made an announcement that all passengers that weren't traveling to LHR needed to stay in their seats for just a moment to let us LHR passengers off. I had absolutely no trouble getting to my gate. It was in the same terminal just down the hall. Boarding was in progress but I zipped to the red carpet and went right on board with no problems.
It's been my experience that the ORD agents won't typically rebook you on ORD-LHR if you are booked ORD-IAD-LHR. The LHR agents will often offer to rebook you on the non-stop back to ORD though.
Halothane