The main issue here is common sense, fares are very easy to look up or estimate, so anyone travelling from an unmanned station can easily know how much their fare will likely be, therefore can plan to have suitable money to pay for it! ie a fare is £2.50 on looking it up, so why would you bring a £20 note or larger to pay for this?!
I agree with several others who said earlier that roving cash handlers (Guards!) can only carry so much float for their own safety, it is nothing like a supermarket till which can hold thousands of pounds before being airlifted to the cash office! Expecting a Guard to have enough change for several people paying with big denomination notes for tiny fares is unreasonable, a bus driver is a more fair comparison!
Although the person mentioned in the OP could be genuine, I have my doubts, they know the system and are trying to play it! So if I was the Guard on that line I would start bringing my own extra float in to ensure this person is taught a lesson, give large notes and you get alot of metal in return, I'm sure he'd start paying with smaller denominations of money when his house fills up with coins!