Will I be registered as a lethal weapon?

NO. Urban legends of people being registered as a lethal weapon with the police once they reach a certain grade and rules around announcing that you practice Karate before defending yourself are all incorrect. Should you injure someone then your position within a Martial Arts club is likely to be considered in court but this is as far as it goes. Also, in a self defence situation, if you need to defend yourself then you do not have time to announce anything as you would be too busy trying to defend yourself. If you have time to announce anything then you have time to remove yourself from the situation without violence.