When I first went to London I bought an oyster card, it was 5 pounds iirc and the lady who sold it told me I could just give it back at the end of my trip and get that 5 pounds back. It's a small extra task to get it and figure it out, but with the tube being as ubiquitous as it is, it's worth investing a bit of time in. It's not like the tickets / ticket machines are any easier.