swanny all the buses have yellow swipe readers, you just touch in once on a bus (no checking out required).
My cousin visited recently, I gave her my debit card to use because she didn't have a contactless card (and because I have a Freedom Pass). We travelled on the Tube from Hammersmith to Covent Garden, then caught a bus to Warren Street area, then back on the Underground to Embankment, walked along the river and across to Southwark, back on the District Line to Hammersmith. I checked a bit nervously - total cost was £6 and some change.