Actually, a citizen of the UK is visa-exempt and does not need a visitor visa to travel to Canada. Once you're married you can accompany her to Canada and speak on her behalf to get her permission to stay with you during processing of the application. You need to follow the instructions that pertain to visa-exempt nationals that's available at the
US2Canada website, on the "Visiting" tab. Pay special attention to the information about Visitor Records. You, as a PR, are required to remain in Canada in order to be eligible to sponsor.
Be aware that it's taking approximately 3-9 months for overseas processing through London, and if an interview is required she will have to attend in London. The inland process, in contrast, takes between 12-18 months to finalize, there is no right of appeal, and applicants should not leave Canada during processing because if they are refused re-entry their inland application is forfeited.
As Leon said, if you were previously common-law partners, you're going to be prohibited from sponsoring her because you did not declare your "qualifying relationship" before you landed. If you were not living together before you immigrated to Canada, be sure that you do not put anything in the sponsorship application that would give CIC reason to believe that you were. Living together is not a requirement for being found to be a "genuine" couple for immigration purposes - so be careful what you include as "proof" of your genuine relationship.
