It is very long time of probation and applying and training, but I think on that occasion it is the best way to do it, that kind of units should be elite, and people who are recruited to work there, should prove their comitment and dedication before hunderds of thoudands pounds will be spend on their trainings.
The only down side is, if you make a decision to take this career path being let's say 25 by the time you get there you're +/- 40
But as I said before that way of recruiting shows how much you really want it.