Carl,
Is there any reason why you are not after joining as a pilot? The reason I ask is that as you see there are fewer career opportunities in aviation after retirement for navs than there are for pilots.
As long as there are pilots to be trained there will be a need for ground instructors at flying schools and this is a role filled almost exclusively by ex-navs.
May I suggest that you aim for pilot and if that does not work at at the selection stage then maybe nav will be offered. Also, should you get selected and fail pilot training the nav route is usually (but not always) offered.
It is, however, very difficult to crossover from Nav to Pilot in the RAF - once they have their teeth into you they don't want to let go.
BTW - the best man at my wedding was a Nav - but then yoiu always want to look cooler than the surrounding company in those photos!