Because victims of domestic violence can be threatened by their attackers (who're seeking to get the case dropped), the policy of the Crown Prosecution Service is rightly that they'll continue with prosecutions (as long as there's sufficient other evidence available) even when a victim indicates that they don't wish the matter to go to court.
https://www.cps.gov.uk/sites/default/files/documents/publications/DV_FAQ_leaflet_accessible_2011.pdf
A conviction for 'grievous bodily harm
with intent' ['Section 18'] will always lead to a prison sentence, with a minimum of 2 years (although the time the offender is actually 'banged up' for is only half of the nominal sentence).
However a conviction for simple 'grievous bodily harm' ['Section 20'] doesn't always lead to an immediate custodial sentence.
So if your partner is only charged under 'Section 20' he stands a reasonable chance of avoiding prison, especially if
(a) his solicitor/barrister ensures that the court is aware of the stress that your partner was under ;
(b) your partner clearly indicates (possibly if/when a pre-sentence report is called for) that he wishes to seek help in controlling his aggression ; and
(c) you make good use of the Victim Personal Statement, that you should be invited to make, to let the court know how you feel.
If you've not yet been invited to make such a statement, there's nothing to prevent you from approaching the police and asking to do so.