Does a parent have to be the one to drop off a child`s uk passport application in bkk?

Hi

My 5 yr old nephew has an appointment tomorrow for a renewal of his uk passport, I was going to go as his mother cant make it and his dad is in the UK at the moment.

Now I am unsure if I can go alone or does his mother HAVE to submit the completed forms in person on his behalf?

Agencies do it as a service so I was hoping that I can basically act as a courier too?

Any thoughts appreciated, thank you