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Zylann said:
Unfortunately she had to return to Canada a while ago, breaking the run of living together

In that case you will need to do a brand new 12 months of cohabitation, or get married.

In the meantime you can certainly travel to Canada as a visitor, and will most likely get a 6 months visitor status. When that time is almost up, you can simply apply to extend it another 6 or 12 months. In this way you could reach 12 months living together to get common-law status.

However you can't work in Canada during this entire time under just a visitor status. Having a gf in Canada does not help you get you any kind of working status. You basically have only 2 options if you want to work:
1. Apply through UK IEC program to get a 1 year open work permit. http://www.canadainternational.gc.ca/united_kingdom-royaume_uni/experience_canada_experience/index.aspx?lang=eng&menu_id=7
2. Find an employer willing to go through the LMO/LMIA process to get you a closed work permit. This is expensive and time consuming for the employer, so is very difficult to get.
 
Do you qualify for a 2 year working holiday visa offered to British citizens? That would be the easiest way for a longer term entry and you can look at your options once already in Canada.

http://www.canadainternational.gc.ca/united_kingdom-royaume_uni/experience_canada_experience/working_holiday-vacances_travail.aspx

However, there's an annual limited quotas so you may have to apply for it at the right times.