If you want to join the RAF as a Navigator, I'm afraid you have already missed the boat (aeroplane?), as the Nav brevet (along with all the other non-pilot ones) has already vanished. The various aircrew specialisations have now all been grouped under the heading WSO/WSOp, depending on whether the individual is commissioned or not.
That said, the job which Navs/WSOs do will probably be around for a while. I'll leave it to a FJ mate to update you on the expected out-of-service date for the Tornado GR (but I believe it's still some way off), but in the ME world the Nav job - if not name - will certainly be around for some time to come, as the Nimrod MRA4 will still have a Nav (albeit only one as opposed to the two which the MR2 carries). There will still be a few other Nav jobs for a little while longer (C-130K, VC-10), but you are right that it is a dying breed, with the rotary world already having turned its back on Navs, and the AD world in the process of doing so.
Have you considered applying for other non-pilot aircrew specialisations?