Surely you need a ticket or tap your card when entering or leaving a station? My husband uses the underground everyday for work and just taps his card. I rarely use cash but I do pay with a card rather than my phone.Slightly off topic but I see a lot of people on the train who have no actual ticket and only buy one on their mobile phone when the Guard or Revenue Officer comes to check tickets. It is giving people an opportunity to only buy when challenged. The railways must be loosing thousands of pounds with people doing this. It's Fare Evasion. And a lot of them play dumb, try using screenshots etc. It takes ages for the inspector to check and sell the tickets. Paper tickets are much easier and there is no need to worry about having enough battery. I put it in my purse and I know it's safe.
I always buy a paper ticket and pay with cash. I like hard currency.