BA 1399 then BA 510 at Heathrow
Belfast City → London → Lisbon
Timetable, London time
16:35BA 1399 from Belfast City at the gate
14:55BA 510 to Lisbon leaves the gate
22 h 20 apart on the timetable, overnight
BA 510 leaves 22 h 20 after BA 1399 arrives on the timetable: more than half a day, so not a connection we count.