A couple of more tips that may or may not be useful:
On the Durham thread in this forum, someone mentioned the phone number 01633 285 422 as direct phone number, but that was back from 2020 so I’m not sure if it’s still valid (+441633285422 in your case, calling from overseas). Someone else in the same thread mentions calling a direct number to the Liverpool office and being able to get more assistance than the general advice line (phone number 0300 123 1973).
Another idea is to send a private tweet to passport office, if you’re on Twitter.