I've read through this thread with interest. I can understand the OP being attracted to the advice - of proactively coming forward - given by the solicitors firm, not least because I imagine they are feeling rather awkward and the aphorism 'honesty is the best policy' might be playing round in their head. However, given the circumstances and the fact that no details were taken (name, address etc), nor was the OP spoken to under caution, it does seem like they'd be incriminating themselves if they just 'fessed up.
Perhaps the punishment here is the feeling of shiftyness and uncertainty.
As others have said, the OP should ensure they are whiter than white from now on.
I'm almost half-tempted to suggest that if they opened an online account and bought their tickets from Chiltern they'd then have a 'paper trail' (or rather digital trail) of their correct behaviour from this moment going forwards, but I suppose the lack of a paper trail before this point in time could also be regarded as suspicious, and I suppose ditto if they suddenly switched from paper tickets to e-tickets.
Instead as a belt and braces I might suggest keeping hold of any paper tickets not swallowed by an automatic ticket gate, likewise keeping a purchase receipt from the TVM or ticket office, but also perhaps keeping them filed away at home rather than retaining them in a wallet or purse used during travel. It's probably an overly paranoid suggestion but the OP could switch to using a different payment card (e.g. credit card rather than bank debit card), or at least get a replacement debit card with a new card number.
However, the suggestions in the last two paragraphs are probably completely redundant to this matter.