There are tons of flights from YYZ-LHR. The hard part is getting to SYD. No business class for all of Jan/Feb 2014, only economy.
I would rather fly via LAX or YVR but I need to stay on BA metal to use my companion voucher. I believe to get to LAX, I would need to use AA.
If it weren't for trying to use up this companion voucher, I would have used Aeroplan to get the shortest flight via YVR.
With the voucher I think you have to fly out of Canada. So that pretty much means Canada to LHR then onward, no flying through US.