Now that buses don't allow cash payments, how does one buy a single ticket on those odd occasions of having come out without the Oyster card? (assuming no contactless debit or credit card)
You don't. You'd have to go to somewhere selling Oyster Cards like a Ticket Stop or Underground station and get another. Or go home again and collect it.
Seriously? There's no way at all of buying a single ticket for a bus any more? That's a bit crap. It's so long since I got my Oyster card that I can't remember the processes for getting one. Where does one get them from if not near a tube station, what's the minimum credit, and how do you get the balance back after the bus journey?