Just to add to what I said above, your best bet is probably an open jaw UK-NYC/YYC-UK, with a single flight YYZ-YYC on either Air Canada or WestJet. Most North American flights are priced on a singles basis anyway and you wouldn't be able to book a YYZ-LHR via YYC.

Alternatively you could see if you can get two separate redemptions YYZ-ORD and ORD-YYC on AA, these would cost you 4500 avios each. Flights from the US to Canada are about a fiver in fees, in the other direction it's about 50 quid (Canadian Airport improvement fee mainly). But this would take significantly longer than just going YYZ-YYC direct.

As to the transatlantic sectors, you are probably best off using ITA matrix, something like outbound LHR,MAN,LBA to NYC and inbound YYC,YVR to LHR,MAN,LBA .