Some fares do not allow stopovers or allow them for a fee. So, you either need to buy up to a fare that allows stopovers or pay the fee associated with the stopover. I think that BA is pricing your trip as 2 through fares XXX-LHR-JFK-YYY(is it MIA)/MIA-LHR-XXX. See how much the domestic sector would cost if booked separably. if it cheaper to buy the domestic leg separately, if would still be possible to but it as one ticket but the agent will need to calculate and store the fares separately.