I flew from EDI-LHR-IAD-IAH on Friday.
EDI-LHR - With BMI, told them my final destination is gonna be at IAH. Standard questions: Has anyone given you anything? etc... However, they could not print UA boarding pass for me from (LHR-IAD, IAD-IAH). But I told them I already obtained my boarding pass through UA online checkin.
LHR-IAD - *A lounge reception agent advised me not to use my paper boarding pass to board since I did not check in at London and direct me to UA connection to obtain UA boarding pass with security sticker.
Monitor displayed "go to gate" status at 10:15am for departure time of 12:05pm. Before getting to gate sitting area, security people searched all bags and everyone was padded down. After I sat down, sure enough, gate agent announced "If you did not check in at London, please go to the desk for passport check". Since I got my UA boarding pass at Terminal 1 UA Transfer desk, that qualified me as checked in at London so I did not need to get my passport check again at the gate.
Got onboard, IFE was working normally with flight map, but no channel 9. We arrived at IAD earlier. Everything was usual, 20 to 25 minutes before landing for landing preparation etc..
Couples changes I experienced, bag searched, padded down, and origin of your check in (additional question). Otherwise, everything else was normal.