If you are in the UK, then the RYA is the way to go. People interested in teaching certification need to do it via the authorized organization in their own country. If you are interested in the best program, the RYA is right up there at the top for programs. It may be expensive, but it seems to be very comprehensive.
Windsurfing schools vary around the world in their requirements for instructors. In the US for example, you need US Sailing certification to teach at most sailing schools in yacht clubs, etc., and to be covered by the major insurance policies. However, there are people teaching who have no certification, but there is no one to stop them from doing so. They are assuming their own risk. In many countries, windsurfing is a "do it at your own risk" sport, and there is nothing to stop you from teaching.
An instructor's course is a good experience in any case, as you learn to rethink how you do everything, and learn to communicate to a wide range of people in a wide range of styles/words/demos/actions. I recommend the training in most any fashion. It will help your own sailing a lot.
hope this helps,