Your definition of 'self defence' might well not be the same the legal one.
If someone attacks you, the only right you have to self defence (in the first instance) is the right to take avoiding action (i.e. to run away).
If it's impossible for you to run away, you then have the right to use the absolute minimum amount of force necessary to avoid further assault. (i.e. you have the right to push the assailant away, giving you the chance to run).
If a simple push won't suffice, you can then use further force but still keeping it to an absolute minimum. Your actions must be solely concerned with preventing further assault upon yourself and not with retaliating in any way. So, for example, you might throw a single punch to the assailant's midriff and then, while the assailant is winded, take the opportunity to run away.
If you used more than the absolute minimum amount of force necessary to prevent further assault on yourself, you're guilty of assault. Any element of retaliation (even if such retaliation consisted of no more than a push) similarly means that you're guilty of assault.
To make 'self defence' stand up in court, you'll need to prove that you couldn't have run away and that you had no choice other than to use the amount of force which you did (strictly in order to avoid further assault and in no way to retaliate).
Chris