I have booked a trip to Oslo on Finnair in Business. After numerous attempts to book online I had to book on the phone with Finnair. Its a Finnair fully flex ticket in I class that allows stopovers in HEL. I paid £601 which was less than ITA Matrix and the online TAs were quoting. I asked if I could book the LHR-HEL sectors under the BA code but couldn't get them to understand what I meant. Does anybody know if this is possible, and if so how to book it that way?

Many thanks