Actually, only J/Y qualify as Business UK on domestic.

There is one other side issue but relating to CE bookings with domestic add-ons, the domestic sectors no longer qualify as Business UK since BA very recently changed the way these through fares are constructed. Now the fare buckets used for the domestic sectors will match those selected for the CE sectors. This will not have any effect on earnings but now when travelling on I, R, C, or D tickets with stopovers you will lose lounge access prior to boarding domestic flights