Studying for a Higher National Diploma affords a good grasp of the basic mechanics of photography as does any serious college course. However, Becki is absolutely right in saying that passionate interest and the will to learn are just as important. The photographers I know like to see a well-presented portfolio of a potential assistant's photographic work, to see if they can put the theory into practice and, also, if they have what it takes to assist and learn from the experience.
As you can imagine, photographers like Paolo Roversi receive many requests from would-be assistants. Don't be discouraged by this. Anything worth doing will attract a lot of applicants. Remember that these are busy people. If you're serious about assisting in Paris - many of the greats live here - then research the photographers in whom you are interested, find out their studio addresses and write them short notes saying that you are coming to Paris after your studies, want to gain experience by assisting and would like to pass by to see them with your book. Keep it short, sincere and fairly businesslike. It is, after all, a job application. Don't gush about how you love their work. It's a major turn-off. Don't gush about yourself. It is permissible to cite inspirations and if their work has inspired you, it's okay to say so. But don't bullsh1t them. Do mug up a bit on their work and their careers. As I said, it's a job application and you'd acquaint yourself with a company to which you were applying, wouldn't you?
In my experience, top notch photographers do make time to see anyone who makes the effort to come and see them. It's the mediocre ones who tend to be arrogant and ungenerous. And you wouldn't want to work for them anyway because they're too miserly to teach you anything!
PK