BA 349 then BA 426 at Heathrow
Nice → London → Luxembourg
Timetable, London time
21:05BA 349 from Nice at the gate
18:40BA 426 to Luxembourg leaves the gate
21 h 35 apart on the timetable, overnight
BA 426 leaves 21 h 35 after BA 349 arrives on the timetable: more than half a day, so not a connection we count.