I think people tend to accept all of this a bit too easily. When I joined the last (UK) company I worked for (I was there for almost 10 years before leaving and becoming an independent expert), they had a similar policy. I simply negotiated business/first class travel as part of my contract, irrespective of this policy. I was quite ready to walk away from the job if they had put their foot down.

Similarly, as an independent I have had clients saying they have Y based travel policies at which point I have drawn their attention to the fact that (a) I am not one of their employees and (b) it's in my T&C that I travel business/first. They are perfectly entitled to take their business elsewhere on that basis but if they don't, that's how it's going to be.

A company/client that only values my employment/service/skills as long as I travel Y is not one I would want to work for in the first place.