A3 877 then BA 633 at Athens
Budapest → Athens → London Heathrow
Timetable, Athens time
19:20A3 877 from Budapest at the gate
16:25BA 633 to London Heathrow leaves the gate
21 h 05 apart on the timetable, overnight
BA 633 leaves 21 h 05 after A3 877 arrives on the timetable: more than half a day, so not a connection we count.