I’m sure they won’t take me if im honest and open right ?
They will take a decision on on whether to prosecute based on their evidence they have which includes what happened on this occasion and any evidence they can find from previous ticket buying etc if they look, which SWR usually seem to do. There is no hard and fast rule either way.
But from what we see on here being honest with them and convincing them that from now on you will always buy the correct fare, increases the
chances that they will not prosecute you.
That's all it's really possible for us to say
Beyond that you can have a read of their policy to give you an idea of what they say they will do:
penalty fares
www.southwesternrailway.com