You still ignore the first sentence which makes it abundantly clear that passengers must buy a ticket before boarding.
If it was there in isolation it would - but it's not, so it doesn't. It explains you must buy a ticket and then goes on to explain what happens if you don't - a more restricted range of tickets is available to you.
To any reasonable person (I tested it on a few people who use rail infrequently and have interest in it like we do) that says you can buy on board but only receive a small range of tickets - meaning it will cost more as a result.
Don't forget of course that when TOC's prosecute they don't prosecute for boarding without a ticket alone. They prosecute for intent to avoid the fare, which is why they tend to ask if you'd have purchased had there been no barrier at the destination station.
Again I must clarify my position - I do not beleive people should be able to buy tickets ontrains without problems if the booking office is open. I beleive they should face prosecution if they do this, BUT this can only realistically happen if its very clear to everyone using the railway what the consequences are.
Currently, its not clear.
What I want is for it to be made clear - not for prosecutions to stop. I find it annoying that I take the trouble to get to the station in good time to purchase a ticket and others don't bother but unlike some in this thread I appreciate much of this is the fault of the railway industry.
How would you like it if I parked my car in your front garden (if you don't have one assume for a moment you do) just because the police would probably not be interested in assisting you?
I have absolutely no idea what relevence you think this has, but if I did so without causing damage to your property you couldn't initially prosecute me for it anyway.