Ok. This response comes with the caveat that none of us here are lawyers giving legal advice but just interested amateurs, and in particular that we are not familiar with the law for buses.
But I think it is probably best to plead guilty.
If I have read the law quoted in post #4 properly, this is a 'strict liability' offence: the regulation doesn't say anything about why you were using someone else's ticket, just that you were caught doing so. As I understand things, you did commit the action, so you have no defence to the prosecution. The magistrate doesn't have to listen to why your case is special, so I would say there is at least a good chance that you will be convicted if you plead not guilty.
If you plead guilty or are found guilty, then you will face a punishment. At worst, this will be a fine (plus court costs, and I would expect compensation (the bus fare) to the bus company). Two things come from this:
- this is the point where the magistrate might be merciful. Does the single justice procedure let you write in your mitigation? If so, use it to tell us what you said in post #6, but provide evidence - maybe a letter from your doctor would confirm what you have said.
- Whether an offence is recorded depends on the maximum punishment (in this case, a fine) rather than what punishment was actually imposed. I hope someone will confirm that I have got this right, but when the maximum punishment is a fine, the offence may be recorded but only for a year (technical point for any one following this - the offence appears to be under a regulation, not a bylaw, and the penalty under act of parliament. So I assume the exemption that we think applies to bylaw convictions doesn't kick in here). So should you face a basic DBS check, this offence will show up for a year, and potentially longer for an enhanced DBS check. But very few organisations demand a completely clean DBS check: they are generally interested in whether you are trustworthy most of the time. One mistake, especially in the circumstances you describe is generally seen as a mistake that anyone could make. What does worry most organisations are attempts to hide this sort of mistake - so if you are convicted, don't try to hide it - it's more likely than not that everything will be ok.