BA 521 then BA 486 at Heathrow
Firenze → London → Barcelona
Timetable, London time
14:05BA 521 from Firenze at the gate
14:00BA 486 to Barcelona leaves the gate
23 h 55 apart on the timetable, overnight
BA 486 leaves 23 h 55 after BA 521 arrives on the timetable: more than half a day, so not a connection we count.