I do not work within the railway industry, however I am an enthusiast and remember going into London from Slough when I lived there a good few years ago and was standing on the train.
Someone from FGW came round to check the tickets, I presume it ws the guard. Anyway, while standing as the train stopped all stations into London this ticket inspector noticed that a seat was vacant said that either I sit in this seat or they would throw me off the train at Southall which was the next station stop.
I complained that I actually had muscle cramp in my legs and would be better standing to release the tension. However, because I did not sit down this ticket inspector and a ciolleague threw me off the train at Southall.
I ended up complaining to the station master at Southall when I eventually found him, who stated to me that the ticket you buy is just for taking you from a to b, it does not cover the price of the seat which if one is available is a bonus. The station master apologised for the way I was treated and said that they should not have thrown me off the train if I was standing minding my own business which I was when I was on the train. I explained to the station master, that as well as relieving pains in my legs I was allowing those more senior to me to be more comfortable.
Morale of the story, is I think it depends on how the guard views the safety of their passengers on the train. That has been my experience anyway.