My reading of the first bit is they are saying you won't get a discount on the £10 fare, e.g. if you have a Railcard.
I'm not asking for more words, I'm suggesting they should be more precise.
The bit about no Railcard discounts crops up in the sentence about "if your fare is already more than £10". They could have easily written it so it applies unambiguously to any situation where you choose not to buy before boarding.
The "exclusions apply" bit is redundant as the exclusions apply in situations where the passenger can't buy before boarding, not situations where they choose not to, so that part of the wording was fine and doesn't need the footnote.
They seem to have tried to rewrite the convoluted NRCoT legalese into plain, simple English. So, for example, "your fare" is a more readable way of saying "the fare for your journey ". It may.be less precise, but the more words, the less people are likely to read something.
But if the passenger knows what the fare normally is and has been accustomed to being able to buy it on board, they're likely to think "your fare" means that fare, and this poster is not doing a good job of telling them that there is a higher fare that would never normally be charged which is what they mean by "your fare".
The idea that there is a higher fare that only applies when you don't pay before boarding is a counter-intuitive concept that needs to be explained if passengers are to have a hope of understanding it. Ascribing the term that is given to that fare in the NRCoT to something else (the minimum fare) and then using the term "your fare" to describe what the NRCoT calls the "full, undiscounted fare", without explaining what you mean, is unhelpful imho.
If you want fewer words and less detail, just say "You are expected to buy a ticket before you board where facilities exist at the station where you start your journey. If you choose not to, you will need to buy a ticket on board. This will be more expensive. See the National Rail Conditions of Travel for further details."