I assume you have already submitted the application to Mississauga and had sponsor approval?
Send the new certificate to CIC London
Canadian High Commission
Immigration Section
38 Grosvenor Street
London
W1K 4AA, United Kingdom
Make sure you include your client ID and all other relevant information so they can match it up with your file.
You could also send a copy as a pdf file attached to a case specific enquiry.
http://www.canadainternational.gc.ca/united_kingdom-royaume_uni/visas/contact_visa_contactez.aspx?view=d