The Gardener writes:

Diabolical. The serious question, though, is "what can you do?" It
seems that complaining to Worst Late Western will only get the "it's
Network Rail's fault" response. Maybe a letter to your MP - letters
from MPs to TOCs tend to get more attention than from the general
public.

Or, get a public spirited, wealthy, philanthropic user of the station to
go to court and obtain an injunction requiring any train which has been
subject to a platform alteration to wait for *all* passenger who are
en-route[1] from the originally advertised platform before departing.

[1] who are proceeding at a reasonable speed taking into account their
age, level of (dis)ability, amount of luggage etc.