Does the TRE really need to go into harm's way, or even go into uniform?
You make a good point. However, when any new fleet comes into being, there is an element of work up. How this will manifest itself in this crossover civ/mil world, I don't know but I can't imagine for a second that a TRE would not be fully operational.
From the bean-counters point of view, if they are paying the TRE £150k (which they'll have to if they want a civvy one), I have no doubt they'll insist on getting their pound of flesh - purely to justify the fact they'll be paying him a similar sum to the PM (that should cause a stir in itself) - including sending him on ops. The airlines certainly do (and rightly so).