VS 12 then BA 854 at Heathrow
Boston → London → Prague
Timetable, London time
09:55VS 12 from Boston at the gate
09:45BA 854 to Prague leaves the gate
23 h 50 apart on the timetable, overnight
BA 854 leaves 23 h 50 after VS 12 arrives on the timetable: more than half a day, so not a connection we count.