I do not know your situation but if passport is of country that does not require visa to enter Canada, then they will request photocopy. (Actually most I have seen make the applicant figure out whether they send photocopy or original, but some offices do things a bit different)
Anyway there is other content of the passport request - again, not from London, so I don't know what they are doing - eg "The processing of your application for permanent residence in Canada is almost complete. You must complete the following steps within 30 days in order for our office to issue your Confirmation of Permanent Residence and, if applicable, your permanent residence visa." [More after trhis]
But it's certainly possible they want a copy of the passport for some reason other than finalizing the file