Do not buy single tickets on the tube, ever. They are designed to discourage you, and to push you towards solutions that are better for TfL and the environment

Do not buy single tickets on the tube, ever. They are designed to discourage you, and to push you towards solutions that are better for TfL and the environment: contactless cards. Get a contactless bank card, Apple or Android pay on your phone or smart watch, or buy an Oyster card. If you know you'll be travelling a lot for a week or two, look into a 7 day travel card which you load onto an Oyster card, and gives you unlimited travel for the zones you're interested in. You can still travel outside those zones, but will pay a fare increment.