BA 853 then FI 451 at Heathrow
Prague → London → Reykjavík
Timetable, London time
12:30BA 853 from Prague at the gate
12:15FI 451 to Reykjavík leaves the gate
23 h 45 apart on the timetable, overnight
FI 451 leaves 23 h 45 after BA 853 arrives on the timetable: more than half a day, so not a connection we count.