Westjet has a sale on flights to London Gatwick (not Heathrow) for $246 out of Toronto (a bit more out of Calgary), returning from $341, totalling around $587 return, taxes included. A similar flight with Air Canada is going for $791. Of course, it's NOT during the summer and has blackout periods.
Here's the details. Click here to be redirected to the Westjet website info on the sale and to book.
July 26, 2016 (11:59 p.m. MT)*
September 6 - December 14, 2016
Offer limited and subject to availability. Advance purchase required.