BA 521 then BA 892 at Heathrow
Firenze → London → Sofia
Timetable, London time
13:45BA 521 from Firenze at the gate
11:10BA 892 to Sofia leaves the gate
21 h 25 apart on the timetable, overnight
BA 892 leaves 21 h 25 after BA 521 arrives on the timetable: more than half a day, so not a connection we count.