Although in London you can just get a free ride if you don't have cash. I see it happen all the time. If you approach the driver and try to pay with cash and say that you don't have a contactless card or oyster card then they just give you a free ride. It seems like a lot of the locals (who probably do have contactless cards and oyster cards) have found out about this and use it to get free travel every day on London buses. At least this is just in London for now rather than the entire country like the Netherlands and Sweden are.
Many places have open boarding. For example most German and Swiss cities. So it is really no different to that.