I think you'll have to regard this as an unfortunate lesson learned. For what it's worth, I agree that you wer enot under any obligation to go looking for the guard to buy a ticket.
With hindsight, it may have been something that would have prevented what occurred later on, but I suspect I would have sat and waited for the staff to come round, just as you did.
Depending on my commitments, I may not have tried to buy a ticket at Brighton either. There ar eplenty of traisn from Brighton, so I may have done if I had some slack in my schedule, but again, there is no obligation to delay your journey in these circumstances.
As far as I can see, where you went wrong is when you tried to correct matters by purchasing tickets after you had received your penalty fare. I agree with MyFriendMary in that respect. So that part of the day's events is the unfortunate bit.