I think there is some confusion here. For short haul plus fares give seat selection at 48hrs. Basic/plus fares are only on one way or simple short haul returns. The OP will not be on a plus fare here.

For the OP's booking seat selection for a non-status passenger will cost up to OLCI and then be free when OLCI opens at 24 hrs. For long haul standard fares seat selection is free from 24 hours.

Just to complicate things, as the OP is on a business fare they may be in J selling class on the short haul, and J selling class does include free seat selection at any time but only for that specific flight.