I was travelling via SWR from Southampton back to London as I always do and have done for the past 5 years. I always buy open return tickets and use them as stated however yesterday I made a mistake. I have dyslexia and attribute this to my mistake of buying a one-way and therefore not having a ticket to show an officer during my return. I began explaining my situation to him and he seemed very friendly and helpful at first. He assumed I was working for a company, and it was bought on my behalf and printed a receipt titled "report for prosecution" after taking my details. I was telling him my situation as he was printing this out and even showed him the trainline app where my purchase history shows many tickets bought in the last few months all open return. I then asked what it is he wants me to sign, and he stated it was for the details to be confirmed as correct. He said that the words on the receipt sound big and scary but it's nothing to worry about really and that they don’t print out tickets and fines anymore and this was the way? He immediately sped off and I was left reading something that says prosecution after never being fined before, currently having a valid railcard that he even scanned and never receiving a warning. Any advice or just pay the £135 in th epost within 21 days like I always read about?
Welcome to the forum.
Although the notice you have been given is headed 'Report for Prosecution', that's about the railway keeping their options open: they may go as far as prosecution, but it's fairly likely that they won't.
Because you were caught without a ticket, the railway could take you to court (in detail - railway byelaw 18(1) says that 'no person shall enter any train for the purpose of travelling on the railway unless he has with him a valid ticket entitling him to travel.' There's nothing here about whether you meant (intended) to do this or not - just not having a ticket is enough to get you into trouble. And byelaw 24(1) says that you can be fined for this. See
https://assets.publishing.service.gov.uk/media/5a79c14b40f0b66d161ade8c/railway-byelaws.pdf for the exact words).
But in general the railway are most interested in (a) getting the fare(s) that they should have been paid and (b) making sure that it won't happen again. So what will happen now is that the railway will have a look at your ticket purchasing record: you've told us that you always buy a return ticket so they won't find anything to worry about - but they will want to get the money back for the journey where you were caught. So they will probably write to you asking for your side of the story: as long as you reply explaining that it was a mistake and you won't do it again, they will most likely offer you an 'out of court settlement' for the train journey you were making, plus the cost of their investigation - so probably the cost of an anytime single from Southampton to London, plus maybe £150 for the investigation.
For someone who had been buying singles and fare dodging for the return journey, the process is much the same - but the railway would be asking for all the dodged fares back. So if you were in that position (and you have told us that you're not) they would be looking to pay quite a lot to the railway.