Panic Button It's quite possible to both plead guilty and be found guilty by a jury on the same facts. If a defendant is charged with two, alternative, counts they may plead guilty to the lesser count but not guilty to the more serious one. They could then be tried on the more serious count and convicted or acquitted by the jury on that.
In this case the woman pleaded not guilty to s18 ( causing gbh with intent to do gbh to someone). That would be count one on the indictment. Count two on the indictment would be s20 ( inflicting gbh).That's an alternative count. She pleaded not guilty to count one, s18, but guilty to count two, s20. The prosecution thought they could prove s18 and so the trial went ahead on that count, the only live issue then being whether she intended to do grievous bodily harm to the man or not since, plainly, she admitted that she had unlawfully caused gbh but did not admit that she intended the victim to suffer gbh when she did so.
If that happened here, the woman can't have been claiming the legal defence of self defence. She can't say 'I'm guilty of gbh s20 assault but was acting throughout in proper self -defence, ' . Using reasonable force in self-defence would be a defence to the s20 so she'd contest that and the s18. My guess is that she said that she set out to defend herself and her child, but accepts that what she finally did went beyond reasonable force, beyond reasonable self defence, and resulted in gbh . Therefore she is guilty of s20, although with mitigation. She denied however that she ever intended the victim to suffer gbh, and so denies the s18 allegation