If they decide to rejoin at some point, perhaps the EU could demand elimination of the pound.
The real sovereignty of a country is not in it's armed forces - its the currency. Without control over the press and interest rates, all you can do is tax and spend in a region with no borders, its like running an oversized city councill. However the amount of solidarity and coordination in EU is not high enough that you can rely on it - you can see that in Dutch ministers refusing to back COVID bonds, and the amount of EU-level economix stimulous being miniscule for the economy size
Without it we literally wouldn't have a lot of anti-eu sentiment in many countries.
No more unelected religious leaders in Parliament, no more unelected upper house at all, and a replacement for FPTP for the Commons.
However, I don't think the Copenhagen Criteria are this specific.
The Guardian speculated in 2013 (http://www.theguardian.com/business/economics-blog/2013/jun/...) that if Britain had joined the Euro the first time:
* The mid-2000s British housing bubble, and its collapse in 2007, would have been even bigger.
* Britain would have had to ask the IMF, ECB, and EU for loans.
* The Tories would in 2010 have promised a referendum to leave the Euro and won a majority. Labour would have won fewer than 100 seats, and UKIP would have "made spectacular gains".
* The anti-Euro side would have won the referendum, and the currency would have collapsed.
* "after a deep and painful recession economic recovery began".
* "Britain would have destroyed the euro on departure, and would now be on the point of leaving the EU altogether. The idea that Farage might be the next prime minister would be quite credible."
I can assure you there is no way Poland adopts Euro, we will sooner Pexit than let Germany manipulate our currency for their profit. Just look what they did to Greece.
Bailed them out of their own short-sightedness, helping to negotiate on Greece’s behalf to force the people Greece/Greek banks owed money to agree to not get it all back, and still helped them as best as possible even though they rejected the terms of the third bailout?
Don’t get me wrong, I don’t claim to know economics well enough to argue how good the Euro is or what reforms it might need to make “The EU bails out Greece” an equivalent statement to “The USA bails out Alabama”, but politically the EU were good on this one.