I don't know how it works in London, but I just moved to Cambridge from Canada. Our contactless cards would not work on the bus system. We had to pay cash until we got our UK banking sorted out. That's not an option on TFL, so visitors just have a plan B if you plan to use your bank card.
If your contactless card doesn't work you can buy an Oyster card from underground stations and most convenience stores which can have a balance added to it with cash.
