Services detail has not seat selection facility. Reply from BA:
"We're sorry that you've been advised different things, David. We've checked your booking and your flight is operated by Iberia Express. We don't operate this route at all. The airline that you are flying with is responsible for booking your seats. We don't even have access to the seat maps. You will need to go back to Iberia and advise them you are flying on their aircraft. We're sorry to disappoint you."

This is the response I got from IB:
Hi David, the flight is not operated by Iberia Express, but by British Airways, we can't assign a seat on a flight we don't operate. You must insist with BA. Regards.

This problem seems to affect a lot of BA/IB/IBX flights UK-Spain. Surely someone at IAG can deal with this...