There is this from the
NRCoC▸ which could be interpreted (as usual, down to interpretation) as relevant in FA's case.
From the
National Rail Conditions of Carriage:
G. TRAIN ACCOMMODATION AND RESERVATIONS
38. Travelling in standard class accommodation with a first class ticket
If you have a first class ticket (or the equivalent) and the first class accommodation (or the
equivalent) shown in the National Rail timetable is not available on any train you travel in,
you may claim a refund of the difference in price between the first class and the standard
class ticket for the relevant part of your journey.
That bit about
'a first class ticket (or the equivalent)' could potentially cover FAs scenario. De-classification makes the 1st Class accomodation
'not available'. In FAs case the difference in fares is the upgrade payment for the
'relevant part' of her journey, which I think should be ^10 as she was in Standard accomodation through two zones. Or
FGW▸ could say that the refund should be based on the zones travelled in prior to the de-classification, in which case it would be ^5.
Either way, I think this part of the NRCoC can be interpreted to cover the de-classification of 1st Class accomodation for refund purposes.
Just my interpretation mind, not a statement of fact!