Canada: Entry by Air

The point of showing an onward ticket is to convince an immigration officer that you have a reasonable plan to leave Canada at a certain time, as part of convincing them that you will really leave Canada when you say you will. So if you can articulate a reasonable-sounding plan to go from Canada to New York and then from New York to London then the ticket will be evidence in support of that plan. Whether the officer will accept your plan as believable and whether they will admit you into Canada is entirely dependent on their judgment. If you would otherwise be admitted, though, I see no reason why flying out of New York instead of out of Canada would be a particular problem -- the long duration of stay is probably more likely to raise red flags, if anything.

Canada (unlike some other countries such as New Zealand) does not have proof of onward travel as a formal entry requirement for visitors.

As others have pointed out, border authorities are looking for people who may overstay either accidentally or deliberately and so proof of onward travel may help overcome that. Proof of a plane ticket from New York likely will help since it is pretty plausible to suggest that you could later arrange some form of transportation to New York.