But if it's a BA metal flight and thus the same plane, how could you not make it? If the inbound to Helsinki is delayed, the outbound back to Heathrow will be, too. They can't start cleaning the plane until everyone is on the bus and headed for the terminal. By the time you walk the five minutes from the bus arrivals gate to the bus departures gate, they should have just started boarding. And they almost always send two buses, so even if you're on the second bus, it shouldn't be a problem. Assuming you're able to do online check-in for the return.