BA 349 then BA 420 at Heathrow
Nice → London → Luxembourg
Timetable, London time
21:05BA 349 from Nice at the gate
14:00BA 420 to Luxembourg leaves the gate
16 h 55 apart on the timetable, overnight
BA 420 leaves 16 h 55 after BA 349 arrives on the timetable: more than half a day, so not a connection we count.