I don’t think you’ll save any money going from Montreal or Boston in this case. Icelandair is restarting Halifax in Spring, so some much-needed competition will return to YHZ transatlantic routes. I used www.itasoftware.com and see fares at $584 USD on Air Canada’s nonstop flight to Heathrow from Halifax. This is roundtrip with all taxes and fees included, and for May outbound from Halifax, I think it’s very good. In fact, I think you should go ahead and purchase at this price—if you can—because I really don’t think it will get any cheaper, but it’s very likely to significantly increase in price. Outbound 17th May, return 19th June, Air Canada flights 860 outbound, and 861 on return. Best to book direct from the airline: www.aircanada.ca

——updated
I found it for $516 USD if you could depart on May 16th. That’s about $611 CDN, not too shabby. I also saw something on AC’s website about a one-day sale to London today only, so I assume these fares will be higher tomorrow. Like I said, if you can snag these today, I don’t think you’ll find anything better.

the one day seat sale is one way tickets only and they ar elike all sold out for the dates i wanna go and then the next cheapest is 919

I used the dates that you gave, and it showed availability, so either they’ve sold out in the last 20 minutes, or one of us is doing something wrong. Try it with www.bookingbuddy.com and click through some of the sources there, like Kayak and Mobissimo, to track it down through them. You might have to pay $5 more to book through one of these sites, but if it helps you find the dates you need, then of course it’s well worth it. Good luck.