Well my current company has started to grow and for some of us travel may be required when it wasn't when we joined. They introduced a formal policy a few months back and I'm not impressed.
Trains: under 4 hours has to be standard class, using advance purchase fares if available.
Flying: under 5 hours Y, over W
Subsistence: breakfast inclusive hotel rate + £25 per night for dinner when an overnight stay. If you're not staying over but needing to leave home earlier or arrive home later due to travel it's only £5 for breakfast and £10 for dinner
It's quite stingy considering what they're earning from us, but I'm not sure if it's deliberate or just put together with someone who doesn't travel much.
Let's use the trains as an example (as currently most of the people who travel are doing short domestic trips for work). A few are doing London - Manchester every week, travelling Monday morning and back on Thursday evening. So they need to get up early, have £5 to spend on breakfast, have to plan to get there early so they don't miss their prebooked train and then have the pleasure of spending a couple of hours on a cramped Pendolino. If they were allowed a first class ticket they'd get lounge access at Euston and Manchester, get fed on the train in both directions, plenty of tea and coffee so they're ready to go when they arrive.
In my case if I was asked to do that I'd rather travel the night before, wake up to a hotel breakfast and I'd be happy with standard class in that situation. Then again using that example LON-MAN I'd be the oddball that insisted on flying.
As for if they asked me to travel internationally if it was occasional and they allowed me to select the airline I'd be happy with their policy. The status benefits and seat selection are critical. I feel trapped in economy in a window seat so always want the aisle unless I'm in row 1 or one of the exit rows on A321.
It's important to remember that any business travel is going to impact on your personal life, it's very unlikely that all your business travel will fit in your work hours and overnight stays take away time with family or friends. Therefore it's only fair that a company compensates you for the extra time, whether this is business class travel, better hotels, overtime payments or in the case of the OP an extra monthly payment.