Just because someone hits you, you've no legal right to hit them back. Even if someone is intent upon carrying out a serious sustained assault on you, you must not hit them unless it's absolutely essential to prevent further injury to yourself. i.e. if you can run away that's what you must do. If you can't immediately run away, but pushing the attacker away will give you the chance to escape, you then have the right to push them but still not to strike a blow. You can only strike a blow if that's the only possible way to prevent further injury to yourself. Even then, you must use the absolute minimum of force necessary.
'Self defence' won't stand up in court, so you might as well forget about trying to use it to justify your actions. Your best hope of avoiding a custodial sentence (which normally results from a GBH conviction) is to engage the services of a good barrister and seek to get the charge dropped to ABH. That's because your account of your partner's injuries indicates that it's a borderline decision between whether the charge should be ABH or GBH.
The definition of ABH, used by the CPS, is here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#09
The definition of GBH is here:
http://www.cps.gov.uk/legal/section5/chapter_c .html#10
Chris