I wasn’t asked on my journey to produce a valid ticket - there was no conductor on the train to buy a ticket.
I was let through the barrier at Ashford and then did what I did at St Pancras. Between Ashford and the officer seeing me at STP, I had not seen a southeastern employee.
Is it likely that I’ll receive a fine and a slap on the wrists?
You keep using the word "fine" and I'm not sure you understand its meaning. A fine can only be imposed by a court.
SE Trains may offer you an out of court settlement, which - to be honest - is the lightest option facing you.
If they choose to prosecute you in court then a fine would be almost certain.
Not sure what you mean by "a slap on the wrists". If you mean a ticking off but no financial implication then I think that's very unlikely.