BA 673 then SQ 319 at Heathrow
Thessaloniki → London → Singapore
Timetable, London time
21:45BA 673 from Thessaloniki at the gate
20:35SQ 319 to Singapore leaves the gate
22 h 50 apart on the timetable, overnight
SQ 319 leaves 22 h 50 after BA 673 arrives on the timetable: more than half a day, so not a connection we count.