Hi,
Not really sure where you got the gen about gaining a civvy ATC qualification automatically through RAF ATC, but unless a lot of things have changed very very recently, I very much doubt it's true. As DJBowl and ProStacker alluded to, it's really retention that's the problem, as opposed to a lack of new blood. If qualifying as an ATCO in the RAF gave you an automatic civvy ticket, that problem would be a whole lot worse! This is because civvy ATC generally earn a lot more than their RAF/RN counterparts, and they don't have to put up with any the "forces BS" (though like any organisation or profession they have their own issues). So people would just join the RAF/RN, get their licences, and then bugger off to civvy street.
What used to be the case was that you could just book yourself in for the necessary exams and boards that qualified you for an ATC licence/ratings, and just turn up and if you passed them then you got the licence. So theoretically anyone could just pitch up to the exams as long as they'd payed the entrance fee! However, nowadays in order to get a Student Air Traffic Controller's Licence you have to undertake a training course at a CAA-approved college, either self-funded (ie you fork out the c.£40k for the course) or sponsored (most commonly through NATS).