BA 759 then BA 638 at Heathrow
Basel → London → Athens
Timetable, London time
17:15BA 759 from Basel at the gate
16:15BA 638 to Athens leaves the gate
23 h apart on the timetable, overnight
BA 638 leaves 23 h after BA 759 arrives on the timetable: more than half a day, so not a connection we count.