They were part of the UK, which is a sufficient reason. But if they weren't part of the UK they still wouldn't have been there. There weren't representatives from lots of countries, it was more about ending a war than having a ceremony. There was a war being fought, right then. The day before the signing the Germans were refusing to surrender and only agreed to after Eisenhower threatened to close the lines to Germans soldiers forcing them to surrender to the Soviets.

There weren't Australian, New Zealand or South African representatives there either. Nor Yugoslavia or Poland. France had lost 5 times the number of soldiers Canada did and was a major power. Canada isn't and wasn't a major power. There were more than a dozen countries that sacrificed as much or more than the Canadians did in WW2. The treaty signing wasn't a reward for coming to war, to treat it as such would have extended the war and cost lives and position. Not to mention the danger and logistics of getting allied representatives to Berlin.

During WWII, Canada was still not fully independent from Britain. Any change to the Canadian constitution before 1981 had to be approved by British Parliament to be in effect, meaning they were still subordinate to the British. Look up the Canada Act.

Essentially your first guess was correct, they were represented at the surrender by the British.

WW2 ended in 1945 not 1940. The Free French forces fought their way through North Africa, Italy, France, Germany before and after the liberation of France.