if you are a visitor to london, to avoid being robbed by tfl - use a uk contactless card and just touch on (and off on the tube) using the yellow touch pads - do not buy a paper ticket from the machines if you don't have a uk contactless card, your best bet is to buy a £3 oyster card, if you want to use public transport quite a lot the tfl website has a lot of info but misses out some vital info - e.g. you can use credit cards in the paper ticket machines at tube stations, but you'll pay over the odds for it.