Have a Canadian Affair
The various meanings of the word "affair" are interesting. Best known, probably, is a romantic affair, followed by a social affair. In railway parlance, the euphemism for an accident is "an affair."
In this case, however, Canadian Affair (www.canadianaffair.ca) refers to low-cost flights that are offered by the company between various cities in Canada and various cities in the U.K. and Ireland during the summer months. I know someone who has travelled on a number of these flights and recommends them for good accommodations and service.
Flights to London land at Gatwick Airpost, which is also the hub for Easyjet (www.easyjet.com,) another budget airline with service to many destinations in Europe and North Africa. If you time your travel right, you might be able to go from a Canadian Affair flight to an Easyjet flight without having to spend a night in pricey London. Ryanair (www.ryanair.com) and Wizzair (www.wizzair.com) are other budget airlines with hubs at different London airports.
A sample price on Canadian Affair for travel roundtrip between Montreal and London in May is $899. Remember, this is in Canadian dollars, so it comes out to about.$810 U.S. at the current exchange rate.