Dear Lara,
If you have qualifications such as O/A levels and a Degree from British institutions you should be OK. An English language O level would be sufficent so that's why it's important to get those duplicates of your certificates.
I provided copies of all such O/A's and Diploma with my application and I have been accepted, the medicals for me and the family are tomorrow! Toronto in the springtime it will be.
The most important area really is the skills that the principal applicant has to offer and the proof of these skills, again trade certificates and references, employer work records and references, etc.
Do you have any blood relatives in Canada, if so you will have to prove the relationships by certified documentary evidence. Key word is proof.
Anyway, good luck and remember if I can do it anyone can
Regards TY.