The contents of this thread are useless to them evidentially, even if they have read it.
They can see where the OP lives, which is presumably in Bath/Oldfield Park, and they can see the OP had bought a lot of tickets from Montpelier to Bristol Temple Meads. They can also see when the tickets were purchased and when the tickets were scanned to exit the station at Bristol.
The fact they stopped the OP gives them all the evidence they need to prosecute for that specific offence. What someone has said under pseudo-anonymity on an internet forum is not evidence.
I suspect GWR will offer a settlement based on the OP’s actual journeys. If there are some journeys where the OP genuinely did travel from Montpelier then the OP will need to explain these, as GWR’s starting position will be all these tickets were used illegally. They probably can’t prosecute for these but then they don’t have to, as they can just prosecute for the incident where they caught the OP red-handed.