"Our tickets were purchased separately, not as one ticket. We can check-in online ahead of time for our BA flight and have our digital boarding passes on us. I was told that we would have to go through security, but wouldn't have to go through immigration because we're transiting from a non-Schengen country to a non-Schengen country. Just wondering if anyone knew if that was correct. Thanks."
Yes, what you are told is technically correct and is 100% true if you had a boarding pass issued at the time of check-in by the airline flying you to Paris. In your situation, assuming that you have a readable boarding pass and do not get shuttled by the Air Canada off the airplane into the immigration/passport control area, you should be able to use the transfer option but there is a real possibility that you will end up at the passport control area and then end up having to exit and then re-enter the departures side. That is what, I believe, Kim is referring to above. You could theoretically go towards the transfer for non-Schengen trips area and would go through security again with your boarding pass. But you could also end up landing at a gate where there is no ability to get to the transfer area because the inbound non-Schengen flight has no need for it because it has no record of people on the flight transferring out of the Schengen zone upon arrival in Paris. It is sort of hard to explain in writing but I hope that I have been clear. The good news is that even if it does not work out for you, you have many options for getting from CDG to London as compared to if your connection was to the longer and more expensive leg of the journey. It's water under the bridge now but in the future when you are doing your own and therefore unprotected connection, you should leave at least 3 hours for the connection even if you do not have checked luggage. At the end of your trip, please report back to let us know what happened.