I do feel for OP but it is strict liability.
I feel for the OP and despair for the majority of people who still seem to think there has been any wrongdoing here. OK, waiver if the OP has not told us the truth, but I'm not sure we'll see them back given the reception they've been given.
Facts: OP usually has a season. Usually arrives on platform 1. Has a bike. Has used platform 2 exit with season ticket before. Is getting daily tickets in July and August because they're cheaper than a season with the 16-25 railcard. Arrives on platform 2 and sees staff checking tickets through the side gate. As this will avoid having to cart the bike over the bridge they ask whether they can buy a ticket. Hand on heart, can anyone really see what is wrong with this scenario?
DaveNewcastle has rightly asked what happened in the 30 minute conversation with the RPI. 30 minutes seems a long time, but maybe it only seemed that long to the OP. If the OP wasn't expecting trouble I can well see how they might seek explanations as to why they are being picked on. Did they fail the 'attitude test'? We may never know. Even if they did, does this change the actual scenario where they needed a ticket at that station and just chose to ask the wrong staff member? Even if they, heaven forbid, became violent, that might trigger an assault charge, but it still doesn't alter the fact that they were not fare evading.
The sign on the gate (or wall next to it) is a bit unfortunate, but is it really a crime to be passing such a sign to get to a member of staff who might be able to save inconvenience of using the bridge? I would suggest that anyone who thinks that a crime has been committed at that point needs to seriously reconsider their attitude to providing customer service.