I did a lot of work in political history and public opinion in graduate school.
From Confederation on, English Canadians typically thought of themselves as "British" in the sense they were part of the Empire. It was their primary political identity. This was not the case with French-Canadians, natives or some groups on non-British immigrants groups. But if you asked a typical person on the street to describe themselves, Britishness would come up quickly. There were liberals with a more pro-US orientation but they were definitely the minority.
As a result, you see huge support for Empire including in the Boer War and the commencement of the First World War. Now during the war, this starts to slowly change more and more. There isn't a sharp break from Empire, but Canada signs the Treaty of Versailles on their own (as does Australia and other Dominions).
Things pick up with the Statue of Westminister establishing more separation from Britain in a legal sense. The beginning of the Second World War sees Canada declare war in its own right and after a short delay from the UK declaration of war.
The final break is Suez. When Britain and France, working with Israel, try to seize the Canal Zone, Canada does not support Britain. This is the first time Canada does not reply to the Empire. In fact, a Canadian diplomat establishes the Peacekeepers who serve to intercede in the crisis, and prevent the British from seizing Suez back.
So there is not a single break but there is a clear shift between 1914 and 1957 that sees Canada's majority political consensus exit "British" primacy and accept "Canadian" identity. It's very messy and complicated, but that's Canada.
Edit: Here are some citations I couldn't do on my phone. Letting the People Decide, Johnston et al; Shock Troops, Tim Cook; Multiculturalism and Nationhood in Canada, Bauhr; Nationalism without Walls, Gwyn; Will of a Nation, Radwanski; Reconciling the Solitudes, Taylor.
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