BA 633 then BA 878 at Heathrow
Athens → London → Bucharest
Timetable, London time
18:55BA 633 from Athens at the gate
18:50BA 878 to Bucharest leaves the gate
23 h 55 apart on the timetable, overnight
BA 878 leaves 23 h 55 after BA 633 arrives on the timetable: more than half a day, so not a connection we count.