The territories each get their own riding for a few legitimate reasons: one, it would be messy to have a rising cross territorial/provincial borders. Two, the north already gets ignored by southern politicians all the time: they have been suffering from food security issues for decades, for example. Three, the "territories" are a large geographic extent mostly comprised of tiny communities of a few hundred to a few thousand people each separated by hundreds of kilometers. If the entire territories were a single riding, the MP for that riding would have a tough time making herself available in person for all her constituents (they have a tough enough time of this already).

Canada hasn't had to use the UK for constitutional amendments since the passage of the Constitution Act in 1982. There is proscribed amendment formula now: the resolution must pass the Commons, the Senate, and be ratified by the provincial legislatures of at least 2/3rds of the provinces comprising at least 50% of the population.