I've personally become much less forgiving of police trying to grab a freebee trip, for a number of reasons:
1. On one of the few occasions I've had to put out a shout for police on-board. I had four MET police on-board, three of whom travelling outside of area and flashing warrant cards. The only one to turn up was the guy who had a valid ticket for the remainder of his journey. So, that kicks the "They are there if you need them" argument into touch for me.
2. I've had to call for Drs or Nurses as often as I have had to call for police on-board. I've never had a Dr or Nurse come up to me and say "can I have free travel, and I'll be in 1st class if you need me". Police are paid considerably more that Nurses, just as an aside, but we expect nurses to pay their fare along with everybody else.
3. I've had a (drunk) police officer attempt to assault me after I charged him up as they plonked himself in 1st with his buddy and said "I'll sit anywhere the f*** I want" when told his buddy's tickets didn't cover him for 1st class, and the PC did not belong to a force that had "free" travel and therefore needed a ticket.
I'm not saying that I've never had cause to ask police who were on-board to assist, and I have been grateful to them for their assistance and made that gratitude known officially, also the regulars I recognise I invite to sit in 1st (especially on late trains).
However, if some bod I don't know from Adam flashes a Thames Valley, South Wales or Avon and Somerset warrant card when I'm going through doing tickets (especially when they are drunk and bugger-all use to me if anything does go pear-shaped), they will be charged.