So the facts are
1 - You went on the tube without enough credit
2 - You did not top up your card knowingly when you knew you don't have credit
3 - Didn't try other means of payment
4 - trying to talk your way out of it because you are a tourist
What am I missing here? Since when being a tourist except someone from paying a fare? And I have never been on a tube station where there are no machine/til to pay. How do you think other people can get in?
You can appeal, they'll just laugh at your face, I mean what is your argument exactly?
Just for the record, I got stung once in Paris and I had a legit case. Just for your information, the exits in Paris tubes don't have a ticket machine, just basically a 1 way gate you just push through. I got myself a pack of 10 tickets, to use on 10 journeys there for the week. I got on, and on one of the connection change I lost it (actually saw it fell out of my pocket and went down the gap in the platform when getting on a train), got off at the Eiffel tower there were 3 inspectors there checking tickets. Now I didn't have a used on on me (a used on have a little ink mark from the machine when you go in). I did have 7 other new ones in my pocket. He wouldn't have any of it, my french is non existent, he gave me a choice of "police or 35 euros" I paid, it was other that of spent the day at a police station and ruin the week.
You live and learn, but you should know better, in fact you KNEW better but ignored it.