BA 673 then BA 1356 at Heathrow
Thessaloniki → London → Saint Helier
Timetable, London time
21:45BA 673 from Thessaloniki at the gate
19:00BA 1356 to Saint Helier leaves the gate
21 h 15 apart on the timetable, overnight
BA 1356 leaves 21 h 15 after BA 673 arrives on the timetable: more than half a day, so not a connection we count.