I flew with BA "the other way" last November, LHR-YVR return.
Doesn't look like I can now see the fare breakdown, but in general terms the taxes, surcharges etc seem to make up the bulk of the fare with very little being attributed by BA to the actual fare that is charged.
It's a bit academic at the moment as BA are not flying to YVR due to covid and travel restrictions, us Brits cannot travel to the USA or Canada even if we wanted to and are clear of the dreaded disease