Skipping a return flight on two way ticket [duplicate]

I suspect there may be a duplicate of this somewhere, but a shallow dive doesn't find it. Regardless the information in the answers here may be relevant. Anyway on to the question.

Blockquote Is this a possible thing to do despite having bought a two way ticket for both our BA and finnair flights?

Practically, as long as you have any necessary visa coverage (based on your dates, you'd seem certain to), then yes, you just don't turn up and get on the first plane and instead check in and get on the new one. In principle, the relevant airlines might shout at you about frequent flier miles, but aren't practically going to do anything about it.

And should we be informing our airline that we won't be showing up for the return flights?

This one is more complicated, Morally it makes sense to announce you aren't travelling, since the airline can resell your seat. However, since airline tickets are sold as a package, this would probably count as a ticket change, which can attract fees and repricing, to the point where your new one way journey costs more than a return, leaving you an additional amount to pay just to get on your first flight. Many people would be willing just not to turn up, and offset the bad karma against the habit of airlines to overbook their planes on the assumption that not everyone will make their flights.