That does not help you even when you are stopped by officer for the first time - it is your responsibility to check and make sure you have a valid ticket for entire distance of your journey. When using Oyster card you do agree to its Terms and Conditions and you must check where to and from you are allowed to use it.As i have stated there is nothing to even tap on
The fact that you've been told that and you still done the same thing the very next day - does not help you here.
Just wait for the letter and take it from there. We don't know what action Southeastern will take on you. Come back to us with what they write to you. Bear in mind that letter might arrive anytime between tomorrow and December.