BA.com can normally handle overnight connections, I do loads of them, though I'm always HBO. But obviously the computer can go sick sometimes, and also for some domestic locations there isn't a set of through fares loaded up, depending on destination. EDI isn't too bad, NCL can be problematic, GLA is in between. Anyway just ring up, if it falls over you won't pay a charge and if there is a complex fares issue they can send it off to the relevant department. And/or try the App (or BA.com if you are booking via the App), if one fails the other sometimes work. Or leave it a few hours and try again. But 2 bookings indeed means no interlining of checked baggage.