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Is there an all-day metro pass in London?
There are several options for unlimited one-day travel on the London Underground including:
Oyster cards have a cap on charges. This means you really don't have to worry at all about what tickets to buy to get the best deal.
Visitor Oyster cards can be bought in advance from outside the UK so that you can arrive in the UK with a ready-to-use Oyster card without having to queue up (stand in line) anywhere. There is a non-refundable £3 fee (unlike ordinary Oyster which have a £5 refundable deposit). There are other benefits, mostly of limited interest.
What is capping?
Capping is a feature of pay as you go which allows you to make a number of journeys in a single day but limits the amount you pay for your travel. Each time you make a journey, you are charged a fare. Once the total cost of all your fares reaches a certain amount you won't have to pay for any more journeys for the rest of the day. This is called a cap.
If you make a lot of rail journeys or a mixture of rail, bus and tram journeys in one day within Zones 1-9, pay as you go with daily capping is better value than buying a Day Travelcard. If you make a lot of bus and tram journeys, pay as you go with daily capping is better value than buying a One Day Bus & Tram Pass.
To benefit from capping, you must touch in and out on every journey using the same Oyster card (touch in only on buses).
I believe you get the same benefits by using a regular contactless credit/debit card or phone. Capping also applies. There should be only one transaction fee per day regardless of the number of journeys.
Both Apple and Android payments should also benefit from daily and weekly capping.
The main alternative to the above is a one-day travelcard which offers unlimited travel within London for a fixed price. I believe this is significantly more expensive than Oyster or contactless.
e.g something like £12.30 instead of £6.60 a day. For offpeak travel in some zones (e.g. 1-6) the difference can be negligible.
For groups of 10 or more people travelling together there are travelcard group options. I think individual Oyster/Contactless are still cheaper and arguably more convenient.