Flying AA from ATL-ORD-LHR.. leave about 630ish am to ORD, with connection to 915A flt to LHR arriving 1020P ( allows me to sleep "normally). Pressing it a little bit.. but no bags to check. However.. cannot check in using kiosk, since I am connecting to intl flight. Very long line of people with lots of luggage, going to Central-South America. Finally get to the front of the line, convince them to get me on..,, race to the gate and get there 8 min before departure. No dice says the gate agent. Off went the flight to ORD without me. Asked for a supervisor, who tried to convince me I should wait for afternoon flight and go overnight to London. I suggested the better plan was to book me on DL to ORD and let me connect to LHR as planned.. which they did. I was pretty incredulous that they denied me boarding while I was holding an Intl flight, but lucked out that DL had a flight that would get me there for my connection. I won't be late for another.. but may choose DL and just do the overnight thing next time.