Well, the flip side of this I suppose is that FCC did offer the option of paying the PF in this instance, and the OP declined to do so. FCC may then argue that they have no other option but to instigate a Byelaw 18 prosecution as a means of getting the revenue owed to them. Playing devil's advocate of course here, but FCC are a wronged party - Greater Anglia sold a ticket to the OP, the OP (accidentally) boarded FCC's train despite not having a valid ticket to do so, and has declined thus far to pay the fare owed to FCC. These indisputable facts should not be overlooked.
Now, I have to say that a Prosecution if this went that far is out of proportion to what the circumstances dictate. I fully expect FCC to accept that the circumstances dictate that a small settlement including payment of the fare should be sufficient to put the matter to bed.
If there is a clarity issue with Anglia's ticket machines, then complaints about that should be addressed to them. Of course, this is why I detest McNumpty's idea of shutting booking offices, but that also is an issue for another thread.
Still, by now, I hope the OP has digested the contents of this thread and is attempting to contact FCC to resolve this unfortunate matter. Having seen previous outcomes, including the chap who was collared for a breach of condition 19, it seems the Head of Prosecutions at FCC is not an unreasonable person, and will hopefully be perfectly inclined to reach a settlement.