CAD179 to upgrade from WT to WTP on the return sector for a YYZ-LHR-YYZ booking. It was mostly additional tax, IIRC. This was shown as a special price, and is indeed much lower than when I checked a couple of weeks ago.

Interestingly, the booking is now in T class and can be changed for CAD275 plus any additional fare, which is the same as before.

Now, to upgrade from WTP to CW for the flight, the cost is a special (for BA!) CAD3,000 one way.