Read through this list:
1. Use of a weapon to frighten or harm the victim
2. Offence was planned or sustained
3. Head-butting, kicking, biting or attempted strangulation
4. Offence motivated by, or demonstrating, hostility to the victim on account of his or her sexual orientation or disability
5. Offence motivated by hostility towards a minority group, or a member or members of it
6. Abuse of a position of trust
7. Offence part of a group action
If two or more of those 'aggravating factors' are relevant to the assault committed by your partner then the offence has passed the threshold for the court to consider a custodial sentence. (Even then, your partner's guilty plea and previous clean record would probably still make custody very unlikely).
If none of those factors apply (or only one does), the bench won't even consider a custodial sentence.
So the chances of your partner being sent to prison are extremely slim. A community order (or even just a simple fine) would seem to be far more likely.
Chris