Thank you for the addition

That is correct and always the case due to the fact that business class departure tax from London is much lower when the destination (for at least 24h) is still in Europe (like KEF is). I however prefer to focus on the best connection times, and the cheaper option you mentioned is only available on FI's site if you specifically search for a stopover itinerary. I am also an advocate of purchasing directly from airlines rather than from 3rd parties.