I once boarded an FGW HST at Bristol Parkway and on opening the door a lad who had been sittong in the vestibule propped up against the door fell out (none the worse for it though) There were plenty of seats available at the time. What is worse than standing is young people like the clown I have just referred to who sit on the floor in the vestibules even though seats available.
Voyagers have plugs just off the floor in the vestibule areas. I'm not sure if there is a reason for having these, did the manufacturers have these people in mind?

This a hundred times. I do it, I've seen others do it (men and women looking at the carriage, seeing one or two seats available but next to someone who looks comfy, look away again).
Standing when there's seats is not the greatest crime against nature, and in any case, if it's down to British character trait, I'd rather it kept than solved.
Some people genuinely don't like sitting next to other people regardless of whether that person is willing to move their bag or not. What irritates me most is people who put their bags on the seat when they can see the service is busy or sit on the isle seat blocking access to the window seat. I've even seen staff travelling like this, sitting on the isle seat at times of peak demand and overcrowding, staff who are most likely to be travelling free-of-cost.
On trains and busses I will put my bag on the seat but if the carriage is busy then I'll make sure it is out of the way so that others are more inclined to make use of the seat. Recently on a XC HST that was packed I was sitting next to a woman and her child she was holding whilst standing. I had a seat next to me but she didn't take it, so I offered her the seat and she was grateful and sat next to me with her child on her lap.
Sometimes I will stand even when seats are available. I'll do this if I am going to need to rush for a connection or if I'm only travelling 2 or 3 stops. I'm quite tall too so sometimes it's nice to get out of my seat and stand for a bit.