Sounds to be a clear case of self-defence to me.
Self-defence is meant to be a reasonable use of force, not excessive force; if one man is punched and he responds by picking up an axe and hacking the other to pieces, that's not self-defence ! Punching him back would be.
Some play might be made of the fact that one party has come off worse, in this case with a broken jaw. But the law applies common sense, and is realistic. At worst, in deciding whether someone used reasonable force the law says that a man cannot be expected, when he comes under sudden attack, to weigh to a nicety what precise degree of force is needed to defend himself ; that he does only what seems to him, in the agony of the moment, to be necessary and reasonable,that is the most compelling evidence that he acted in self defence. In other words, the fact that this came off worse in these circumstances, even with a broken jaw, doesn't mean that is not self-defence.
Two punches, whatever the consequences, were certainly reasonable here, on your account. The same could be said of three or more, if necessary to stop a battering from him.
Incidentally, it's doubtful whether s18 will be persisted with. The injury is quite serious but it's difficult to prove the necessary intent to cause it.It isn't the necessary consequence of a couple of punches. Ordinary gbh, s.20, is more appropriate. But whatever the charge ends up as, on your account you have a defence of self-defence to it.