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If you don't want to use an Oyster card or travelcard when travelling in London, or simply haven't had time to pick one up yet, you can pay cash for each single journey. But remember, this is more expensive than using an Oyster card.
When travelling on the tube, you can buy your ticket using cash at the station you're travelling from. You can use a ticket machine or go to the ticket office, where there is a station attendant.
When buying your tube or train ticket, you might find it easier and quicker to use a ticket machine.