BA 581 then BA 672 at Heathrow
Milan → London → Thessaloniki
Timetable, London time
13:55BA 581 from Milan at the gate
10:25BA 672 to Thessaloniki leaves the gate
20 h 30 apart on the timetable, overnight
BA 672 leaves 20 h 30 after BA 581 arrives on the timetable: more than half a day, so not a connection we count.