UA 929 then BA 680 at Heathrow
Chicago → London → Istanbul
Timetable, London time
05:55UA 929 from Chicago at the gate
18:10BA 680 to Istanbul leaves the gate
12 h 15 apart on the timetable
BA 680 leaves 12 h 15 after UA 929 arrives on the timetable: more than half a day, so not a connection we count.