I've got a trip booked through BA, YYZ-LHR on BA, LHR-HEL-ARN on Finnair. I'm silver so I expected to be able to select my Finnair seats for free. I can't actually select them at all. Is this normal?
Perhaps try retrieving your BA booking on the Finnair website? Ensure the FF nbr is added there and it should allow you to assign your seating.