Used to that you were entitled to Club Class if there were no RAF seats or flights available.. Also the flight had to be over 2 1/2 hours.
But now with budget holders etc - its down to you to see if you can convince them that you need to travel Club class (and if they can afford it) or wait for a service flight if there is one. Time away from a desk is not a valid reason! (Can't remember which JSP it comes from . . . JSP327??)
Have seen flights to Gib where the Stn Cdr was in cattle class but an RN AB was in Club class as all the charter seats had been filled by the time he was booked. (UK - GIB schedueled time 2 hrs 40 mins)