"He will still get the chance to put in a defence , it is called mitigation."
Beware Eddie, the two are not the same. If you defend a matter you plead Not Guilty and (usually) put forward a different version of events to that presented by the prosecution. It's then left to the court (jury or the Magistrates) to decide whether you are guilty or not. When you present mitigation you accept the allegation (and so plead guilty) but put forward reasons why the offence was committed in an effort to reduce its seriousness.
Anyway, back to the question, as Eddie rightly points out, GBH with intent is a serious offence which always carries a custodial sentence - sometimes a lengthy one. It is the most serious offence of assault there is and the only was an assault can be more serious is if someone is killed. Your partner's solicitor is the best person to put your questions to.