Every ticket has to be a return ticket.

FORBES

Shashi Verma, TfL's director of customer experience, said: ''Pay as you go Oyster cards do not expire and customers are able to return their cards to a Tube ticket office at any time for a refund of the remaining balance and card deposit.

BBC: Unused Oyster cards valued at ?55m

Sadly only available in Japan for the time being, so you'll need to include a return ticket in that splurge too.

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