If you read the Penalty Fares Rules you will see that there is written into them the provision that, if you do not successfully appeal, or pay the sum claimed by the notice within 21 days, the TOC may cancel that option and proceed to prosecution of the original offence, which may be:
a) Fail to pay the fare and obtain a valid ticket before boarding a train when pre-purchase facilities were available contrary to National Railway Byelaw 18.2 (2005)
b) Fail to show a ticket contrary when asked to National Railway Byelaw 18.2 (2005)
c) Travel on a railway without having previous