There is also no requirement to be issued with a penalty fare. It's not a case of 'must'. They are given for honest mistakes or acts of carelessness at the discretion of revenue staff. As in law at least you avoided paying the correct fare deliberately (your actions match those of someone who does this)* a penalty fare is not really relevant. It's doubly not relevant as you didn't travel on a penalty fare service. The good news is that as others have said an out of court settlement is likely.
*Most fare dodgers will pay if challenged. But they will take actions exactly like yours to try and 'chance it' without payment. As regards to whether it'll be a byelaws or regulation of railways act case, you'll have to wait for the letter. They'll probably go for the former (certain to succeed) but if they go for the latter it will probably succeed too. Intent in law is judged by words and actions, not what goes on inside your head. But don't worry too much. East midlands trains seem to settle out of court quite often. Either way I'm sure you've learned the value of buying before you board.
When it comes to responding to the letter be apologetic and factual, not defensive. There should be a few main points: what you did (without further incriminating yourself or making excuses), sorry and it won't happen again, an offer to pay off any costs and the fare outstanding, and once again sorry. Whilst it would not ordinarily be necessary to get a solicitor involved in a relatively simple case such as this, if you know someone who is one who is willing to help you, I'd say so much the better.
