I hope/assume that the usual exceptions for no ticketing facilities available would apply. However, as the majority of stations don't have any facilities for selling excess fares, only TVMs that can sell ordinary tickets, this would open a lorry-sized loophole in the regulation. As others have said, it will be instructive to see what the new conditions actually say.
This thread is timely:
Off-peak to Anytime excess requirements
I am sure a great many excess fares that are owed go unpaid. Probably the overwhelming majority. That is not passengers' fault, it is train operating companies' fault entirely.
Criminalising failure to do the impossible before travelling is obviously unjust.
Without seeing the draft text it's impossible to know how this will interact with the rules on ticketless travel. It would appear there is a desire to treat situations where an excess is due but hasn't been paid before boarding as if they were the same as situations where no ticket is purchased before boarding. To me, that shows a quite stunning lack of thought and/or a want of understanding by the people involved. The situations are nothing alike.
Most of the time, most tickets you might want can be obtained either online or from a TVM before boarding. Excesses usually can't be.
Maybe the intention is simply to make it impossible, in most situations, to avail of the right to excess a ticket, thereby forcing people to buy an entirely new ticket. And at the same time preventing them getting a refund on the ticket they already held. So double-charging, in other words.
Looking at it like that, I'm not only wondering if Lord Hendy will be signing these changes off: I'm wondering if he may actually have banged them out in one of his furious emails to his officials?