BA 677 then BA 638 at Heathrow
Istanbul → London → Athens
Timetable, London time
16:40BA 677 from Istanbul at the gate
16:15BA 638 to Athens leaves the gate
23 h 35 apart on the timetable, overnight
BA 638 leaves 23 h 35 after BA 677 arrives on the timetable: more than half a day, so not a connection we count.