Don, as a non-American who pays attention, I think I can help answer your question.

I'm not sure a national referendum, as such, is technically possible in the U.S. The United States is 50 sovereign states, a federal district and some territories, all flying in a formation that is not necessarily close, guided by the Executive Branch of the federal government and the Supreme Court in compliance with the Constitution. That's why they have an Electoral College; the national popular vote for the Presidency is tallied by state and it is the states' weight in the Electoral College that determines which candidate wins.

Canada is a federation of provinces under a Westminster-system parliament. We could do it -- in fact, we have done three national referenda, the most recent in 1992 over the proposed Charlottetown Accord -- because the federal government is the boss of us all. Australia and New Zealand -- ditto, with minor variations.

I believe the United Kingdom would actually have a hard time doing a binding national referendum. Technically, I think the vote would have to be conducted separately in England and Wales, Scotland, and Northern Ireland.