+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
You need to order the certificate from this website
www dot acro.police.uk/police_certificates.aspx

I ordered mine prior to applying for my student visa.
 
Very easy. Apply online. Fill the form, upload your photo and documents and wait for it to come in the post.
 
http://www.cic.gc.ca/english/information/security/police-cert/europe/united-kingdom.asp