Try booking with the AA ATW desk:
+1800 247 3247

The AA ATW desk is generally more competent than other oneworld airlines in organising these things. Less surcharges too. Call weekday mornings Dallas time for best results.

According to the rules you'd be able to get the UK fare if the ticket is purchased in Canada. So you should be able to get your itinerary priced at the UK fare by telling the ATW desk that you're intending to purchase the ticket in Canada. Give 2-24 hours for the itinerary to get priced.

Now I have no actual experience of issuing a ticket in Canada, but you should be able to purchase the ticket by fronting up at the nearest AA ticketing counter (at an airport I'd imagine), or ringing up the reservations hotline for Canada if your itinerary is eticketable (not more than 16 segments, no open dated coupons).