No cash - London buses have been cashless since 2014. You can top up Oystercards at tube stations and at any shop displaying "Oyster Ticket Stop" - these will mostly be newsagents. Even easier, if you have a contactless debit or credit card, that will work so you have no topping up to think about.
To make this work, all bus trips are one flat fare of £1.50. There is now also a Hopper fare - if you get on any more buses within an hour, they are free, and the electronics will work all that out for you. So no extra fares if you need to change buses to get to where you want to go. You don't need to do anything about this, you just get it automatically.
Which gives me a great shopping cheat - there are a number of supermarkets 10 minutes by bus away from me, and the bus goes every 10 minutes. If I can whizz round a supermarket in half an hour or so, I get an automatic free ride home. Good, innit?