FWIW, the Westjet offer seems to have been replaced by "Perkopolis Flights by Voyzant".
Similar to Perkopolis access to Travel & Lesiure Go hotel stays, this appears on the surface to access wholesale fares not available to the general public.

I just did a sample search for a YVR-LHR itinerary for random February dates & compared to Expedia.
Both returned the same fare.