You will need a National licence to compete on the roads, which means you will already have a reasonable amount of experience by the time you attain this.

After that it's really down to when you feel ready to give it a go, generally the roads tend to have a natural steadying influence on your riding, certainly I remember my first visit to Scarborough as a rider and I was in no doubt after practice that I couldn't afford to get anything seriously wrong!!

Hope this helps.