UPDATE: This from Strawbrick
The "1st Class" reminder in the door lobby may not be that pointless.
It is actually "against the rules" to be in a designated 1st class area without a 1st class ticket whether or not you are occupying a 1st class seat, i.e. if you are standing. It has been like this for years, for example it applied to the corridors outside any 1st class compartments. (You were of course permitted to pass and re-pass through 1st class sections to get to the Restaurant Car or to look for a seat at the other end of the train.)
Bearing in mind the possible penalties for not having a 1st Class ticket perhaps the sign is actually not that pointless.
Of course, if the train is full and standing the Conductor / Guard / Train Manager can, at his / her discretion, make an announcement that he / she has declassified one or more 1st class sections (and that 1st class ticket holders can apply for a refund of the excess).