on the tap-on / tap-off things on the trains and buses. You will want to use an Oyster Card. They cost GBP 5 up front then you just load them up with cash at pretty much any train station. If you want, I've got three Oyster cards from our last trip I could send you so you would avoid having to buy them over there. They don't have any value left on them, but you'd avoid the up front purchase price.