UA to UA xfers @ IAD can allow for very fast connections. The problem is its easy to get a slight delay on both sides, quickly eating into your connection time. If possible, I'd try and standby an earlier ORD -> IAD flight (there are plenty) or try and fly standby on the ORD -> LHR non-stop.
BearX had a good point. Is this an all UA itin or is an AA/UA mixed?