Why would they give discretion to tourist ?? Then everyone would claim to be one ! Thats not test is not fare dodging or not, you travel without paying, that's the line. Simple as.
If the police caught you driving without insurance, but it was a Sunday, and you claim you can't get through to them on the phone, you'd think that's an excuse?
You can justify it how you want, bottomline. You knew about not having credit, you travel without credit and got caught, you can't prove you tried to pay online. You think Winona Ryder would've use "I have millions of dollars, I don't need to steel" in her defence? Of course not. And you using "I can afford to pay for it, but there were no means of payment" is the same. Millions travel each day and managed to pay their ticket without problem, I've been on some really dodgy stations, and trust me, there is ALWAYS a machine, takes both CARD and CHANGE. And I don't live in london either.
Please do use "I am a tourist" in your appeal, they'll prob pin it up on the Staff room pin board with it highlighted