If you really have no time to buy an Oyster Card try to borrow one from a friend or relative and pay them the appropriate amount in cash. It cannot be a Young Person's or 60+ Oyster Card, as those are non-transferable.
Then you literally can't travel... as that's how it works now with TfL.
You need an Oyster card, contactless payment card, or travelcard. No driver takes cash; and no ticket/payment option means no travel.
You have all day now, go and get an oyster card. You cannot get on a bus without one or contactlsss payment. There's no magic answer.
Go into virtually any small shop or station and buy a One Day Bus Pass for ￡5. It will take you just 1 minute
www.tfl.gov.uk
(Tank please not you cannot pay cash on buses or buy tickets from bus conductors in London)
www.tfl.gov.uk
Set out earlier to catch the bus and give yourself time to get a card
Use cash and buy a ticket from the conductor.
You have all night, find an atm and get some cash
