Stamping and processing are different things. Processing of FSW applications for gcc and pakistan happens in London, but for gcc the applicants are directed to send their passports to abu dhabi for visa stamping. Pakistanis may request for their visa to be stamped in islamabad as well. The visa stamping in Abu Dhabi is clearly mentioned in the passport request email:
"Send your passport(s) along with the required documents listed above by courier to the following address:
VPU Processing (LDN)
Immigration Section
Embassy of Canada
10th Floor, West Tower
Abu Dhabi Trade Towers
P.O. Box 6970
Abu Dhabi, United Arab Emirates
NOTE: You must include a fully completed Aramex airway bill or a self-addressed C4 or C5 envelope. These must include your complete name, physical address (no PO Box), and phone number (country code + area code + phone number) If you do not write your address CLEARLY on your return airway bill, your passport(s) may be lost in transit. "