This will be a class 150/2 I expect. That door immediately behind the cab at each end is built as a passenger door, and when operating as a 4 car set (as an inner cab) will still function as a passenger door.
Unions however wanted the guard to have his own vestibule, so when they were a couple of years old 'private' was painted onto the outside and the electrics were altered.
I don't agree with it, personally.
I also think it is disgraceful to eject passengers in this manner and then leave them behind - whether they've had a drink or not.
Let us not forget that the main USP of trains and buses is that passengers can go have a drink then not worry about drinkdriving.