You would have to call BA to sort it out. Had the same experience last year when trying to book the direct flight that Wideroe operates on behalf of Finnair from BGO to HEL. I did however check on finnair plus whether there were any available redemptions before I called. You could always book LHR-HEL-OSL and then fly onwards with SK or DY, but OSL-TOS return would probably set you back somewhere between £150 and £170.