Passport collection

I have two of my children's first UK passport in Bootle HMPO. I applied using standard service. I expect their passports will be printed sometime this week. As we're travelling on the weekends this week, I won't have time to wait for delivery. I've seen some timelines on here showing that they have collected the passport as soon as it has been printed from HMPO. How can I go about collect my children's passports from HMPO as well? What comes to mind is as soon as I get the email that the passport has been printed, I just show up at HMPO and collect it. But what if it's too late and they have already passed it on to the courier? Many thanks.