They already were part of such an union. They are still members of the EFTA.
The EFTA was an association of countries based on the same goal as the EU but meant to become a confederation established on Free Trade principles. In fact there were 3 unions in Europe in the 60s : the EEC, the Comecon and the EFTA. The members of the EFTA were UK, all Scandinavian countries, Switzerland and Austria. The GDP of that area was very near to that of the EEC, and ahead that of the Comecon.
Many people from the countries from that association hope to mold the EU into what the EFTA was. I think this is impossible with France and Germany inside EU.

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From SR