This is not true. It is illegal for pakistani passport to travel without the person only from Pakistan. This is a pakistani law. Otherwise you can send ur passport anywhere as long as you are not sending it FROM pakistan.
And as for his concern, u can send it from Canada nd receive it there too. Just email them nd let them know of your situation. I know this cuz there was a paragraph in my passport request about this, pasted below:
"If you will want your passports returned to an address outside of Pakistan, then this letter does not apply to you. Inform us immediately so that we can send you different instructions."
I have been reading this issue and I will say my close friend got her skilled worker class immigration from foreign country and that time immigration office was Islamabad and she sent her passports to Pakistan in courier and provided her Pakistani address to send passports. After that no courier company was able to send passports from Pakistan to her .At the end they spoke consulate General of Pakistan in the that foreign Country and make arrangements and got their passports back through embassy.
By the way if she is in Canada situation is different as visa exempt country people mostly got their approval while living in Canada as well.
I have two suggestion
1- please do not waste your time and talk to Pakistani Consulate/London office and send your passports.
2-Talk to good immigration lawyer who deals with mostly Pakistani case he might suggest you something better.
I wish this should be simple as explain your situation to London office and send your passport to London and receive it back in Canada.
Thanks