Look at them as one-ways, and fly Canada-London-Continent and then Continent-London-USA for the cheapest trip. Why it works that way??? Beats me. But $3100 RT should be feasible, given the following examples from 4/12:
YYZ-BOS-LHR-BCN (AC/BA/BA) - $1771
YYZ-BOS-LHR-ARN (AC/BA/BA) - $1913
YHZ-BOS-LHR-ARN (AA*/BA/BA) - should come out to $1760, but Expedia gets confused if you toss a commuter flight in there...
ARN-LHR-BOS (BA/BA) - $1289
BCN-LHR-BOS (BA/BA) - $1465