And what happens if I ask for a zero-fare excess there?
Clue: I have never, not once, been issued one even where I have specifically requested it. I've even ended up with bizarre workarounds like refunding an already used ticket as if it hadn't been used and issuing a replacement (this one was care of Euston LM a couple of years ago). But mostly been told not to worry as the fare is the same.
A prosecution in this case is beyond ridiculous.
That's my experience too. The LM guard who found London-Atherstone was a zero-fare excess over London-Nuneaton for me didn't issue one, he just told me not to worry.