You are on the early morning departure to IAD... you'll have no problem getting through immigration and customs at the midfield terminal and make your connecting flight at that time of the day. It can get crowded when the afternoon arrivals from Europe all converge at IAD around the same time, but in the morning there are very few UA arrivals and you will not have any problem. I did this morning LHR-IAD flight in June and it was a ghost town in there. I was through immigration and security and heading toward my gate in under 10 minutes (no checked bags). With checked luggage it'll be a little longer but no reason to worry.