For me Kayak tend to be the best, Its like a comparison website site for flights.
Just be a little bit wary, some of the cheaper flights are classed as ultra economy and only include hand luggage. They don't include checked baggage or food & drink. With Westjet a checked bag returned is normally $100 CAD on top of flight costs. If you want checked baggage make sure you go to filters and tick checked buggage. Makes comparison easier and helps avoids any hidden charges.
You'll be able to find a return flight for roughly £500/£600 a person with luggage.