London, United Kingdom
Never pay for a single trip on the tube. The prices are very expensive. The easiest is if you have a contactless bank card, then you can use it just like Oyster card, but you don't need to buy Oyster and don't need to top up. If you don't have one of those, and staying in London just for a few days, then you can get a One Day Travel Card. Check in which zones are you planning to travel, but usually 1-3 should be enough. The you can travel with it as much as you want. And if you are staying longer, then get an Oyster card, which is pay-as-you-go, but it has a spend cap, meaning that it won't charge you more than a certain amount (for zones 1-3 it's £7.70 per day)
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