I feel so stupid!

_ollie_

I think you should have researched a bit more on what you would get.

Example: southernrailway.com/tickets-and-fares/ticket-types/first-class-travel/
Which says:

Southern offer First class tickets on the majority of our routes. If you choose to work on the train, there are power points close by on some of our services.
When catering facitilites are available, First Class passengers are entitled to a complimentary tea, fair-trade coffee or small mineral water.


So they don't claim that it will be available on every train, thus you would be unlikely to be successful in any claim.