Seems fairly straightforward to me on the Air Canada site using the multi-city option: LHR-xYYZ-YQT-YYZ-LHR. Base fare works out at GBP316.77 for some random dates I tried, compared to GBP294 for a simple roundtrip to Thunder Bay without the stopover in Toronto.
The taxes and surcharges are huge though: the total fare is GBP515.80! But I guess there's not much you can do about them.