Why the fuss?
The penalty fares rules are straightforward - if there is no working TVM▸ or open ticket office at the station where you started your journey, you cannot be charged a penalty fare.
End of story.
It's also just occurred to me that this is a "Fares Fair" thread, rather than "Across the West" one, so I'll shift it over...
So how many people know that's the case?
They will all have read in the Daily Mail that if go anywhere near a train without a ticket you get charged an horrendous penalty.
Why not just say publically that if the ticket macines will is switch off between T1 and T2 then penalty fares will not apply in this period. preferably on a notice near teh TVM.
Taplow station will be unmanned from around 13:00 on 24/12 so presumably the machine will be switched off a full 8 hours before deadline unless someone is going to go round on Xmas Eve at 20:00 to switch all the machines off.
It's "Attention to Detail" and not making your passengers more anxious than they probably are trying to make a journey on Christmas Eve.