Thank you for your response, I will not be home for around a week due to work and I'm worried that the letter might come during that period in around 2 months, is there a way to know when the letter has arrived? Also what are the chances of them not offering a settlement, in which case will I get a record if I'm taken to court? I've just been completely mortified by this all and would appreciate all the advice I could get. Is it worth getting a solicitor? Apparently they can reach out to GTR before the letter arrives, but I'm also worried about the amount I may need to pay as I am not paid a whole lot, I'm just really confused at the moment
In general 'the letter' will allow you two or three weeks to respond, so being away from home for a week shouldn't be a problem - although if it does arrive while you're away you will need to deal with it promptly as soon as you get back. But while the railway have up to six months to write to you, from what we've been seeing GTR (the Thameslink company) tend to be much quicker than that: you will probably (not certainly) hear from them in the next few days.
GTR quite often offer an out of court settlement, but we can't tell you how likely this is to happen: you have told us that you have done this 'multiple times' and I imagine it will make quite a difference if 'multiple times' means that you have done it twice or if it means that you have done it several hundred times.
And you probably don't yet need to involve a solicitor: if GTR are prepared to settle out of court then you won't need one - it's only if you are taken to court (and the letter will either have a date on it for a court appearance or talk about the 'single justice procedure') that you will need to think about hiring a solicitor.
It's worth going through the worst that can happen. That would be that you are prosecuted under the Regulation of Railways Act, you are convicted and punished. The punishment will be a fine, plus court costs, plus prosecution costs, plus compensation to the railway. This will add up to quite a lot. But you will not be sent to prison.
Also in this worst case scenario, your offence will be recorded by the DBS. But for most purposes, your record will be clear after a year - this is because of the Rehabilitation of Offenders Act which says that a fairly minor criminal conviction should not be allowed to hang over someone's life forever.
But for some purposes you may always have to declare it. Those purposes include some jobs where a particularly high standard of trust is needed - things like being a lawyer or an accountant or working in health care. Also (at least arguably) if you want to visit the USA and use the visa waiver program. But none of these need ruin your life: for working in healthcare, the regulator (NMC, GMC etc) may decide that fare-dodging is <edit> not </edit> relevant to your health care abilities, and to get a visa to go to the USA you might have to take a day off to attend an interview at the American embassy to prove that you are the sort of person they're prepared to let into their country.
So if the worst happens, you may have to pay quite a lot of money (almost certainly more than you have saved by fare-dodging). You may face some embarrassing interviews. But more likely than not your life and career won't be ruined.