Along with airlines like JetBlue and Bamboo Airways, WestJet has managed to secure slots at London Heathrow. WestJet was given 732 total slots and they will fly (just like JetBlue) from Terminal 2.
The Heathrow website shows a 9 a.m. arrival and 11 a.m. departure for Vancouver service and 11:20 a.m. arrival and 1:20 p.m. departure for Calgary service. No comment has been made about service to Toronto as of yet. The flights will be operated by the WestJet 787-9 Dreamliner. Flights will be able to start as early as May 1.
I think that the A330-800 would be easy to buy since the Hawaiian Airlines deal got hijacked by Boeing and there are white tail A330-800s sitting around