FB 851 then BA 854 at Heathrow
Sofia → London → Prague
Timetable, London time
16:55FB 851 from Sofia at the gate
10:40BA 854 to Prague leaves the gate
17 h 45 apart on the timetable, overnight
BA 854 leaves 17 h 45 after FB 851 arrives on the timetable: more than half a day, so not a connection we count.