Save this for your next trip to london. if you are wanting to use the subway in london. Don't buy a ticket from the ticket machines. Those machines are extremely confusing and you will end up spending more than you need to or might not buy the right ticket and have problems exiting the subway. Instead use a contactless (or tap pay) card to scan into and out of the subway. This makes sure you always pay the right amount. Plus, if you ride several times, when you hit the cap for the day (about 8 pounds if you are staying inside the city) than it will stop charging you beyond that.