The Byelaws state,
"“valid ticket” means a ticket...entitling that person to use the particular railway service he is using or attempting to use."
How certain can we be that a higher court would not interpret "service" as implying transport as far as Epsom, since the OP did not deny that was his intention?
Consumer law favours the consumer's interpretation of ambiguity in a contract. But is that enough of a guarantee for the OP here?
Or does "service" clearly mean service to any station, rather than specifically to Epsom?
The mention of the word "boarding", as differentiated in other laws and conditions from "travelling" (or remaining on board), makes it quite clear that the Byelaw is concerned with those who fail to make use of available ticketing facilities to buy any ticket at all.
I don't think it is supportable to claim that service can be interpreted to mean journey, by any means - many people's journeys involve taking more than one service; it would be perfectly legal for the purposes of the Byelaws to board your first train on a multi-leg journey, with a ticket that only covered you to your first interchange station, and then to buy a ticket onboard this first train to cover the rest of your journey. In other words, you do not need to have a ticket that covers your entire journey merely to
start it.
The prosecution is far from having a slam-dunk case as may have been taken as being implied by previous posts. I don't think specialist advice is necessary by any means, given the relatively less serious nature of the prosecution (i.e. it isn't a case for long-term fraud of thousands of pounds), but the OP may wish to avail themselves of an initial consultation with a criminal defence solicitor, which is usually free or very low cost.