Unfortunately, you have no right or entitlement to the course, they're offered as a gesture of goodwill from the police. You may enquire politely and they may reconsider but given the timing, they will likely not allow this as they'd want to avoid the offence becoming time-barred (6 months from the date of the offence the case needs to be laid with the courts). I'd also not delay in replying to the FPN as the closer it gets to the 6 months the more likely they are to just go to court wherein the punishment can increase.