The problem with the view that the EU should be a simple free trade organization is that this does not lead to a stable system.
In fact, to fix the problems that arise with free trade among EU member states, one needed the euro. Not that it doesn't introduce more problems itself.
When a group of trading partners decide to open their borders and reduce tariffs to 0, gaming starts.
There is the temptation to manipulate your currency to gain advantage, which is what happened until the exchange rate mechanism was put into place. This limited the maximum swings in rates to +/- 5% over an established base rate. This was the "snake in the tunnel."
This does not work in a world market in which those same currencies, subject to supply and demand pressures, may be valued by the market above or below the official, artificial rate targets. Keeping currencies within the exchange rate mechanism became a game between central banks and currency traders. And it locked in rates that over time did need to change more than the 5%.
So the time came to either eliminate the exchange rate mechanism altogether, which leaves no solution for a free trade block moving forward, or go to the euro.
To dream of a Common Market of gentlemanly trade among EU nations using national currencies is to dream of a return to a broken system. The choices today are (1) move forward (2) forget it (3) a reduced EU of countries with similar economies.
(3) would be better, with an initial euro zone of Benelux, Germany, France, and Austria, plus those Scandinavian countries interested. Southern and Eastern Europe should have been asked for another 20-50 years development before being allowed to join.
I do not think the UK should be part of the EU, or the euro at least, due to deep cultural factors.