Don’t complain to DHL, complain to His Majesty.
I'm going through this process now myself... what is the logic behind needing to send a photocopy of every page of my other passports? It seems like such a silly waste of paper.
Just to provide an update on British passport turnaround times, they appear to have cleared their backlog.
I submitted my passport renewal application on 27th October, and received the new one today. Turnaround time including delivery of 12 working days. I have never said this before in my life, but for once a British government office demonstrated efficiency!