First, let's look at the 'self defence' claim:
If someone attacks you the only legitimate 'self defence' (in the first instance) is to remove yourself from the situation. (i.e. to run away). If that's impossible, it's then legally permissible to use minimal force to give you the chance to remove yourself from the situation. (e.g. you can push the attacker to the floor and then run away). If running away is clearly impossible, you can use such force as is necessary to protect yourself, but no more. Based upon your account, your ex has absolutely no chance of proving 'self defence'. Saying "I though she was going to hit me, so I hit her first" won't stand up in court. He'll be asked why he didn't simply run away (as the law requires).
Courts are coming under pressure to imprison everyone who simply carries a knife. Fortunately, the courts are resisting these pressures because not everything is 'black' or 'white'. However, anyone who uses a knife (even if it's not actually used to stab or slash the victim) is almost certain to face a lengthy custodial sentence. The magistrates have recognised this as they've referred the matter to the Crown court because they believe that their maximum sentencing powers (i.e. 6 months imprisonment) are unlikely to be sufficient.
The maximum penalty for ABH is 5 years imprisonment (plus an unlimited fine). I'd guess that your ex is probably looking at around a 2 year sentence, with half of that time actually being spent 'inside' .
Try thinking of the court hearings as the chance 'to get your own back'. Feeling vindictive is rarely positive but I feel that your case may well be an exception ;-)
Chris