If you appreciate fairly priced non-stop flights have a look on this Black Friday week sale from the UK to Canada based on Air Transat. They now offer discounted non-stop flights from London or Manchester to Vancouver from £386. You can also fly on a direct route London–Calgary from £391. This is for a roundtrip including all taxes and fees, meals as well as checked luggage. (Which is no longer a standard on flights between Europe and USA / Canada). Moreover there is also some good availability in summer season incl. beginning of summer holidays 2019..
Air Transat promotion offers the cheapest flights from 15th of April to 4th of July 2019. Based on latest date of your outbound flight from the UK to Canada. You will find some good deals from January to March as well however not a non-stop flights as this is a seasonal service available in summer season. Max stay 1 month. Promotional sale is running till 26th of November or until sold out. You can book your accommodation in Vancouver or Calgary via Booking.com or Hotels.com.
Travel dates is the easiest to compare directly at website of Air Transat as the cheapest option vary by each of the selected routes.
Booking sample of cheap non-stop flights from the UK to Vancouver or Calgary from £386! As per above link to the deal you would fly from London Gatwick to Vancouver on 11th of May. Return from Canada back to the United Kingdom on 24th of May 2019.