BA 821 then BA 892 at Heathrow
Billund → London → Sofia
Timetable, London time
14:00BA 821 from Billund at the gate
11:10BA 892 to Sofia leaves the gate
21 h 10 apart on the timetable, overnight
BA 892 leaves 21 h 10 after BA 821 arrives on the timetable: more than half a day, so not a connection we count.