No... You shouldn't forget an undertaking. With the courts experiencing an avalanche of these hearings, the Magistrates themselves are now increasingly looking to see if an undertaking may be quick & affective means of dealing with these cases that warrant it...Forget an undertaking. My understanding is they just don't happen.
Going against you though is the fact that this actually involved a physical assault... I Imagine much will hinge on the risk assessment, but, that should not stop you from seeking first the possibility of an undertaking being an acceptable compromise. It sounds like your partner would be agreeable, then depending on that assessment, you may find the magistrate actually suggesting that the police prosecutor also accept it.... Certainly nothing to be lost by trying that approach first