Graham O,
There seems to be a slight hint of green eyed monster in your post.
So to reassure you, and to set your mind at rest, short haul staff tickets are about the same price as advance booking full fare tickets when all the taxes etc are added on.
Your argument that if staff paid full fare prices would be cheaper does not hold water.
There can be a fair old saving on long haul but you are assuming that staff would travel anyway. If it is discretional travel most would not.
Also, even on long haul many staff would pay full fare to travel when they want to in order to book accommodation in advance, or to ensure a seat when they need it i.e. to weddings, funerals etc.
The main advantage of staff travel is the ability to buy tickets at the last minute or to cancel at the last minute without penalty.