Mark Brader wrote:

So why is this allowed? I have no problem if the train operators want
to use their first-class rolling stock this way, but it seems obvious
to me that they should be required to mark it as second class, instead
of requiring passengers who don't buy first-class tickets to know
which trains the timetable says first class exists on.

Once upon a dim distant time paper window labels were used to indicate
declassified accommodation. However, that died out during the British Rail
days (around the mid to late 1970s). Other than on the Midland Main Line
Meridians, where there are 'permanent' window labels in one vehicle,
declassifying standard class, I don't recall seeing it in many years. I
suspect that it fell out of favour with higher diagramming of stock,
resulting in stock working on several routes in a day, some of which may
require first class accommodation, and the manpower was not there to keep
labelling and de-labelling stock. However, IIRC, most of the Windsor line
diagrams are captive and stock could, theoretically, be labelled up at the
start of the day and remain so until end of service.